https://wiki.xenproject.org/index.php?title=Virtualization_Spectrum&feed=atom&action=historyVirtualization Spectrum - Revision history2024-03-28T20:32:37ZRevision history for this page on the wikiMediaWiki 1.31.3https://wiki.xenproject.org/index.php?title=Virtualization_Spectrum&diff=12965&oldid=prevRcpavlicek at 23:17, 13 October 20142014-10-13T23:17:45Z<p></p>
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</table>Rcpavlicekhttps://wiki.xenproject.org/index.php?title=Virtualization_Spectrum&diff=12964&oldid=prevRcpavlicek at 23:14, 13 October 20142014-10-13T23:14:41Z<p></p>
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</table>Rcpavlicekhttps://wiki.xenproject.org/index.php?title=Virtualization_Spectrum&diff=12963&oldid=prevRcpavlicek at 23:13, 13 October 20142014-10-13T23:13:33Z<p></p>
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</table>Rcpavlicekhttps://wiki.xenproject.org/index.php?title=Virtualization_Spectrum&diff=11586&oldid=prevLars.kurth at 12:42, 9 May 20142014-05-09T12:42:18Z<p></p>
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</table>Lars.kurthhttps://wiki.xenproject.org/index.php?title=Virtualization_Spectrum&diff=6020&oldid=prevPsr520 at 20:40, 23 January 20132013-01-23T20:40:05Z<p></p>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>At XenSummit 2012 in San Diego, Mukesh Rathor from Oracle presented his work on a [http://vimeo.com/49506288 new virtualization mode], called “PVH”. Adding this mode, there are now a rather dizzying array of different terms thrown about — “HVM”, [[Paravirtualization_(PV)|“PV”]], [[PV_on_HVM|“PVHVM”]], “PVH” — what do they all mean? And why do we have so many?</div></td>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>At XenSummit 2012 in San Diego, Mukesh Rathor from Oracle presented his work on a [http://vimeo.com/49506288 new virtualization mode], called “PVH”. Adding this mode, there are now a rather dizzying array of different terms thrown about — “HVM”, [[Paravirtualization_(PV)|“PV”]], [[PV_on_HVM|“PVHVM”]], “PVH” — what do they all mean? And why do we have so many?</div></td>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>The reason we have all these terms is that virtualization is no longer binary; there is a spectrum of virtualization, and the different terms are different points along the spectrum. Part of the reason the terminology is a little unclear is the history; any language and terminology evolves over time in response to the changing situation. However, changing the terminology after-the-fact once certain usages become common is difficult.</div></td>
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<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>The reason we have all these terms is that virtualization is no longer binary; there is a spectrum of virtualization, and the different terms are different points along the spectrum. Part of the reason the terminology is a little unclear is the history; any language and terminology evolves over time in response to the changing situation. However, changing the terminology after-the-fact<ins class="diffchange diffchange-inline">,</ins> once certain usages become common<ins class="diffchange diffchange-inline">,</ins> is difficult.</div></td>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>So in this article I will introduce just enough history to understand how the current situation came about, and (hopefully) introduce a consistent set of terminology which may help clear things up, while balancing this against the fact that people will still continue to use existing terminology.</div></td>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>So in this article I will introduce just enough history to understand how the current situation came about, and (hopefully) introduce a consistent set of terminology which may help clear things up, while balancing this against the fact that people will still continue to use existing terminology.</div></td>
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<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>This article will give a general introduction to virtualization, and to [[Paravirtualization_(PV)|paravirtualization]], Xen’s unique contribution to the field, as well as the advent of hardware virtualization extensions (HVM). It will also introduce the idea of adding paravirtualized drivers for disk and network and cover the motivation and technical descriptions of two more modes which further mix elements of full virtualization and paravirtualization.</div></td>
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<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>This article will give a general introduction to virtualization, and to [[Paravirtualization_(PV)|paravirtualization]], Xen’s unique contribution to the field, as well as the advent of hardware virtualization extensions (HVM). It will also introduce the idea of adding paravirtualized drivers for disk and network<ins class="diffchange diffchange-inline">,</ins> and cover the motivation and technical descriptions of two more modes which further mix elements of full virtualization and paravirtualization.</div></td>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>In the early days of x86 virtualization, all of this needed to be virtualized: disk and network devices needed to be emulated, as did interrupts and timers, the motherboard and PCI buses, and so on. Guests needed to start in 16-bit mode and run a BIOS which loaded the guest kernel, which (again) ran in 16-bit mode, then bootstrapped its way up to 32-bit mode, and possibly then to 64-bit mode. All privileged instructions executed by the guest kernel needed to be emulated somehow; and the pagetables needed to be emulated in software.</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>In the early days of x86 virtualization, all of this needed to be virtualized: disk and network devices needed to be emulated, as did interrupts and timers, the motherboard and PCI buses, and so on. Guests needed to start in 16-bit mode and run a BIOS which loaded the guest kernel, which (again) ran in 16-bit mode, then bootstrapped its way up to 32-bit mode, and possibly then to 64-bit mode. All privileged instructions executed by the guest kernel needed to be emulated somehow; and the pagetables needed to be emulated in software.</div></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td class="diff-marker">−</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>This<del class="diffchange diffchange-inline"> is</del> mode<del class="diffchange diffchange-inline">,</del> where all of the aspects the virtual machine must be functionally identical to real hardware<del class="diffchange diffchange-inline">,</del> is what I will call fully virtualized mode.</div></td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>This mode<ins class="diffchange diffchange-inline"> -</ins> where all of the aspects the virtual machine must be functionally identical to real hardware<ins class="diffchange diffchange-inline"> -</ins> is what I will call fully virtualized mode.</div></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>== Xen and paravirtualization ==</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>== Xen and paravirtualization ==</div></td>
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<td colspan="2" class="diff-lineno">Line 65:</td>
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<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>At the same time the Xen team was coming up with paravirtualization, the engineers at Intel and AMD were working to try to make full virtualization easier. The result was something we now call HVM — which stands for “hardware virtual machine”. Rather than needing to do software emulation or binary translation, the HVM extensions do what might be called “hardware emulation”.</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>At the same time the Xen team was coming up with paravirtualization, the engineers at Intel and AMD were working to try to make full virtualization easier. The result was something we now call HVM — which stands for “hardware virtual machine”. Rather than needing to do software emulation or binary translation, the HVM extensions do what might be called “hardware emulation”.</div></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td class="diff-marker">−</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>Technically speaking, HVM refers to a set of extensions that make it much simpler to virtualize one component: the processor. To run a fully virtualized guest, <del class="diffchange diffchange-inline">may</del> other components still need to be virtualized. To accomplish this, the Xen project integrated qemu to emulate disk, network, motherboard, and PCI devices; wrote the shadow code, to virtualize the pagetables; wrote emulated interrupt controllers in Xen; and integrated ROMBIOS to provide a virtual BIOS to the guest.</div></td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>Technically speaking, HVM refers to a set of extensions that make it much simpler to virtualize one component: the processor. To run a fully virtualized guest, <ins class="diffchange diffchange-inline">many</ins> other components still need to be virtualized. To accomplish this, the Xen project integrated qemu to emulate disk, network, motherboard, and PCI devices; wrote the shadow code, to virtualize the pagetables; wrote emulated interrupt controllers in Xen; and integrated ROMBIOS to provide a virtual BIOS to the guest.</div></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Even though the HVM extensions are only one component of making a fully virtualized VM, the “fully virtualized” mode in the hypervisor was called HVM mode, distinguishing it from PV mode. This usage spread throughout the toolstack and into the user interface; to this day, users generally speak of running a VM in “PV mode” or in “HVM mode”.</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Even though the HVM extensions are only one component of making a fully virtualized VM, the “fully virtualized” mode in the hypervisor was called HVM mode, distinguishing it from PV mode. This usage spread throughout the toolstack and into the user interface; to this day, users generally speak of running a VM in “PV mode” or in “HVM mode”.</div></td>
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<td colspan="2" class="diff-lineno">Line 83:</td>
<td colspan="2" class="diff-lineno">Line 83:</td>
</tr>
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<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Most modern operating systems need just two levels of protection: user mode and kernel mode. Kernel mode memory is protected from user mode memory via the pagetable “supervisor mode” bit.</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Most modern operating systems need just two levels of protection: user mode and kernel mode. Kernel mode memory is protected from user mode memory via the pagetable “supervisor mode” bit.</div></td>
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<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<tr>
<td class="diff-marker">−</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>When running a virtual machine, you need at least three levels of protection: user mode, guest kernel, and hypervisor. The hypervisor memory needs to be protected from the guest kernel, and the guest kernel memory needs to be protected from the user. The pagetable protections only provide two levels of protection, so Xen uses another processor feature, called a segmentation limit, to provide the third level of protection. Segmentation limits are processor feature that was in common use before paging was available. But since paging has been available, segmentation limits have basically not been used; so Xen was able to commandeer them to provide the extra level of necessary protection. The pagetable protections protect both the guest kernel and Xen from userspace; the segmentation limits protect Xen from the guest kernel.</div></td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>When running a virtual machine, you need at least three levels of protection: user mode, guest kernel, and hypervisor. The hypervisor memory needs to be protected from the guest kernel, and the guest kernel memory needs to be protected from the user. The pagetable protections only provide two levels of protection, so Xen uses another processor feature, called a segmentation limit, to provide the third level of protection. Segmentation limits are<ins class="diffchange diffchange-inline"> a</ins> processor feature that was in common use before paging was available. But since paging has been available, segmentation limits have basically not been used; so Xen was able to commandeer them to provide the extra level of necessary protection. The pagetable protections protect both the guest kernel and Xen from userspace; the segmentation limits protect Xen from the guest kernel.</div></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Unfortunately, at the time that Xen team was developing clever new uses for this little-used feature, AMD was designing their 64-bit extensions to the x86 architecture. Any unused processor feature makes hardware much more complicated to design, reason about, and verify. Since basically no operating systems use segmentation limits, AMD decided to get rid of them.</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Unfortunately, at the time that Xen team was developing clever new uses for this little-used feature, AMD was designing their 64-bit extensions to the x86 architecture. Any unused processor feature makes hardware much more complicated to design, reason about, and verify. Since basically no operating systems use segmentation limits, AMD decided to get rid of them.</div></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td class="diff-marker">−</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>This may have greatly simplified the architecture for AMD, but it made it impossible for Xen to squeeze in 3 levels of protection into the same address space. Instead, for 64-bit PV guests, both guest kernel and guest user-space need to run in ring 3, each with their own address space. Every time a guest process needs to make a system call, it has to bounce up into Xen, which will context-switch to the guest kernel. This not only takes more time for each system call, but requires flushing one of the key CPU caches, called a TLB. Frequent flushing of the TLB causes<del class="diffchange diffchange-inline"> the</del> all<del class="diffchange diffchange-inline"> of the</del> execution to run more slowly for some time afterwards, as the TLB is filled up again.</div></td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>This may have greatly simplified the architecture for AMD, but it made it impossible for Xen to squeeze in 3 levels of protection into the same address space. Instead, for 64-bit PV guests, both guest kernel and guest user-space need to run in ring 3, each with their own address space. Every time a guest process needs to make a system call, it has to bounce up into Xen, which will context-switch to the guest kernel. This not only takes more time for each system call, but requires flushing one of the key CPU caches, called a TLB. Frequent flushing of the TLB causes all execution to run more slowly for some time afterwards, as the TLB is filled up again.</div></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>In 64-bit HVM mode, the problem doesn’t occur. The HVM extensions make it easy to have three different protection levels without needing to play clever tricks with little-used processor features. So making system calls in 64-bit HVM mode is just as fast as on real hardware. For this reason, a lot of people began running 64-bit Linux in fully virtualized mode.</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>In 64-bit HVM mode, the problem doesn’t occur. The HVM extensions make it easy to have three different protection levels without needing to play clever tricks with little-used processor features. So making system calls in 64-bit HVM mode is just as fast as on real hardware. For this reason, a lot of people began running 64-bit Linux in fully virtualized mode.</div></td>
</tr>
</table>Psr520https://wiki.xenproject.org/index.php?title=Virtualization_Spectrum&diff=5534&oldid=prevLars.kurth at 21:07, 4 November 20122012-11-04T21:07:33Z<p></p>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>{{sidebar</div></td>
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</table>Lars.kurthhttps://wiki.xenproject.org/index.php?title=Virtualization_Spectrum&diff=5533&oldid=prevLars.kurth: Added part 22012-11-04T20:38:42Z<p>Added part 2</p>
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<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>{{TODO|Redefine Glossary to point to definitions on this page}}</div></td>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>{{sidebar</div></td>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>The reason we have all these terms is that virtualization is no longer binary; there is a spectrum of virtualization, and the different terms are different points along the spectrum. Part of the reason the terminology is a little unclear is the history; any language and terminology evolves over time in response to the changing situation. However, changing the terminology after-the-fact once certain usages become common is difficult.</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>The reason we have all these terms is that virtualization is no longer binary; there is a spectrum of virtualization, and the different terms are different points along the spectrum. Part of the reason the terminology is a little unclear is the history; any language and terminology evolves over time in response to the changing situation. However, changing the terminology after-the-fact once certain usages become common is difficult.</div></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td class="diff-marker">−</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>So in this <del class="diffchange diffchange-inline">series of articles</del> I will introduce just enough history to understand how the current situation came about, and (hopefully) introduce a consistent set of terminology which may help clear things up, while balancing this against the fact that people will still continue to use existing terminology.</div></td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>So in this <ins class="diffchange diffchange-inline">article</ins> I will introduce just enough history to understand how the current situation came about, and (hopefully) introduce a consistent set of terminology which may help clear things up, while balancing this against the fact that people will still continue to use existing terminology.</div></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td class="diff-marker">−</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>This article<del class="diffchange diffchange-inline"> will be divided into two parts. The first part</del> will give a general introduction to virtualization, and to [[Paravirtualization_(PV)|paravirtualization]], Xen’s unique contribution to the field, as well as the advent of hardware virtualization extensions (HVM). It will also introduce the idea of adding paravirtualized drivers for disk and network<del class="diffchange diffchange-inline">.</del> <del class="diffchange diffchange-inline">The second mode will</del> cover the motivation and technical descriptions of two more modes which further mix elements of full virtualization and paravirtualization.</div></td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>This article will give a general introduction to virtualization, and to [[Paravirtualization_(PV)|paravirtualization]], Xen’s unique contribution to the field, as well as the advent of hardware virtualization extensions (HVM). It will also introduce the idea of adding paravirtualized drivers for disk and network <ins class="diffchange diffchange-inline">and</ins> cover the motivation and technical descriptions of two more modes which further mix elements of full virtualization and paravirtualization.</div></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>== Full virtualization ==</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>== Full virtualization ==</div></td>
</tr>
<tr>
<td colspan="2" class="diff-lineno">Line 71:</td>
<td colspan="2" class="diff-lineno">Line 71:</td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Once you have a fully-virtualized system, the first thing you notice is that the interface you provide for network and disks — that is, emulating a full PCI card with MMIO registers and so on — is really unnecessarily complicated. Because nearly all modern kernels have ways to load third-party device drivers, it’s a fairly obvious step to create disk and network drivers that can use the paravirtualized interfaces. Running in this mode can be called fully virtualized with PV drivers.</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Once you have a fully-virtualized system, the first thing you notice is that the interface you provide for network and disks — that is, emulating a full PCI card with MMIO registers and so on — is really unnecessarily complicated. Because nearly all modern kernels have ways to load third-party device drivers, it’s a fairly obvious step to create disk and network drivers that can use the paravirtualized interfaces. Running in this mode can be called fully virtualized with PV drivers.</div></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>== From Poles to a Spectrum ==</div></td>
</tr>
<tr>
<td class="diff-marker">−</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>== Summary ==</div></td>
<td colspan="2" class="diff-empty"> </td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>I have introduced the concepts of full virtualization and paravirtualization (PV), as well as the hardware virtualization (HVM) feature used by Xen (among other things) to implement full virtualization. I have also introduced the concept of installing paravirtualized drivers on a fully virtualized system.</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>I have introduced the concepts of full virtualization and paravirtualization (PV), as well as the hardware virtualization (HVM) feature used by Xen (among other things) to implement full virtualization. I have also introduced the concept of installing paravirtualized drivers on a fully virtualized system.</div></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td class="diff-marker">−</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>This small step, from full virtualization towards paravirtualization, begins to hint at the idea of a spectrum of paravirtualization.<del class="diffchange diffchange-inline"> In part 2,</del> I will <del class="diffchange diffchange-inline">cover</del> the historical reasons for the development of PVHVM, and finally of the newest mode, PVH.</div></td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>This small step, from full virtualization towards paravirtualization, begins to hint at the idea of a spectrum of paravirtualization. I will <ins class="diffchange diffchange-inline">continue with</ins> the historical reasons for the development of PVHVM, and finally of the newest mode, PVH.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>== Problems with paravirtualization: AMD and x86-64 ==</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>It comes as a surprise to many people that while 32-bit paravirtualized guests in Xen are faster than 32-bit fully virtualized guests, when running in 64-bit mode, paravirtualized guests can sometimes be slower than fully virtualized guests. This is due to some changes AMD made when designing the architecture which simplified things for them, but made things more difficult for Xen.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>Most modern operating systems need just two levels of protection: user mode and kernel mode. Kernel mode memory is protected from user mode memory via the pagetable “supervisor mode” bit.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>When running a virtual machine, you need at least three levels of protection: user mode, guest kernel, and hypervisor. The hypervisor memory needs to be protected from the guest kernel, and the guest kernel memory needs to be protected from the user. The pagetable protections only provide two levels of protection, so Xen uses another processor feature, called a segmentation limit, to provide the third level of protection. Segmentation limits are processor feature that was in common use before paging was available. But since paging has been available, segmentation limits have basically not been used; so Xen was able to commandeer them to provide the extra level of necessary protection. The pagetable protections protect both the guest kernel and Xen from userspace; the segmentation limits protect Xen from the guest kernel.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>Unfortunately, at the time that Xen team was developing clever new uses for this little-used feature, AMD was designing their 64-bit extensions to the x86 architecture. Any unused processor feature makes hardware much more complicated to design, reason about, and verify. Since basically no operating systems use segmentation limits, AMD decided to get rid of them.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>This may have greatly simplified the architecture for AMD, but it made it impossible for Xen to squeeze in 3 levels of protection into the same address space. Instead, for 64-bit PV guests, both guest kernel and guest user-space need to run in ring 3, each with their own address space. Every time a guest process needs to make a system call, it has to bounce up into Xen, which will context-switch to the guest kernel. This not only takes more time for each system call, but requires flushing one of the key CPU caches, called a TLB. Frequent flushing of the TLB causes the all of the execution to run more slowly for some time afterwards, as the TLB is filled up again.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>In 64-bit HVM mode, the problem doesn’t occur. The HVM extensions make it easy to have three different protection levels without needing to play clever tricks with little-used processor features. So making system calls in 64-bit HVM mode is just as fast as on real hardware. For this reason, a lot of people began running 64-bit Linux in fully virtualized mode.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>== Paravirtualizing little by little: PVHVM mode ==</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>But fully virtualized mode, even with PV drivers, has a number of things that are unnecessarily inefficient. One example is the interrupt controllers: fully virtualized mode provides the guest kernel with emulated interrupt controllers (APICs and IOAPICs). Each instruction that interacts with the APIC requires a trip up into Xen and a software instruction decode; and each interrupt delivered requires several of these emulations.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>As it turns out, many of the the paravirtualized interfaces for interrupts, timers, and so on are actually available for guests running in HVM mode; they just need to be turned on and used. The paravirtualized interfaces use memory pages shared with Xen, and are streamlined to minimize traps into the hypervisor.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>So Stefano Stabellini wrote some patches for the Linux kernel that allowed Linux, when it detects that it’s running in HVM mode under Xen, to switch from using the emulated interrupt controllers and timers to the paravirtualized interrupts and timers. This mode he called PVHVM mode, because although it runs in HVM mode, it uses the PV interfaces extensively.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>(“PVHVM” mode should not be confused with “PV-on-HVM” mode, which is a term sometimes used in the past for “fully virtualized with PV drivers”.)</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>With the introduction of PVHVM mode, we can start to see paravirtualization not as binary on or off, but as a spectrum. In PVHVM mode, the disk and network are paravirtualized, as are interrupts and timers. But the guest still boots with an emulated motherboard, PCI bus, and so on. It also goes through a legacy boot, starting with a BIOS and then booting into 16-bit mode. Privileged instructions are virtualized using the HVM extensions, and pagetables are fully virtualized, using either shadow pagetables, or the hardware assisted paging (HAP) available on more recent AMD and Intel processors.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>== Problems with paravirtualization: Linux and the PV MMU ==</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>PVHVM mode allows 64-bit guests to run at near native speed, taking advantage of both the hardware virtualization extensions and the paravirtualized interfaces of Xen. But it still leaves something to be desired. For one, it still requires the overhead of an emulated BIOS and legacy boot. Secondly, it requires the extra memory overhead of a qemu instance to emulate the motherboard and PCI devices. For this reason, memory-conscious or security-conscious users may opt to use 64-bit PV anyway, even if it is somewhat slower.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>But there is one PV guest that can never be run in PVHVM mode, and that is domain 0. Because having a domain 0 with the current Linux drivers will always be necessary, it will always be necessary to have a PV mode in the Linux kernel.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>But what’s the problem, you ask? Weren’t all of the features necessary to run Linux as a dom0 upstreamed in Linux 3.0?</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>Yes, they were; but they are still occasionally the source of some irritation. The core changes required to paravirtualize the page tables (also known as the “PV MMU”) are straightforward and work well once the system is up and running. However, while the kernel is booting, before the normal MMU is up and running, the story is a bit different. The changes required for the early MMU are fragile, and are often inadvertently broken when making seemingly innocent changes. This makes both the x86 maintainers and the pvops maintainers unhappy, consuming time and emotional energy that could be used for other purposes.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>== Almost fully PV: PVH mode ==</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>A lot of the choices Xen made when designing a PV interface were made before HVM extensions were available. Nearly all hardware now has HVM extensions available, and nearly all also include hardware-assisted pagetable virtualization. What if we could run a fully PV guest — one that had no emulated motherboard, BIOS, or anything like that — but used the HVM extensions to make the PV MMU unnecessary, as well as to speed up system calls in 64-bit mode?</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>This is exactly what Mukesh’s PVH mode is. It’s a fully PV kernel mode, running with paravirtualized disk and network, paravirtualized interrupts and timers, no emulated devices of any kind (and thus no qemu), no BIOS or legacy boot — but instead of requiring PV MMU, it uses the HVM hardware extensions to virtualize the pagetables, as well as system calls and other privileged operations.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>We fully expect PVH to have the best characteristics of all the modes — a simple, fast, secure interface, low memory overhead, while taking full advantage of the hardware. If HVM had been available at the time the Xen hypervisor was designed, PVH is probably the mode we would have chosen to use. In fact, in the new ARM Xen port, it is the primary mode that guests will operate in.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>Once PVH is well-established (perhaps five years or so after it’s introduced), we will probably consider removing non-PVH support from the Linux kernel, making maintenance of Xen support for Linux much simpler. The Xen kernel will probably support older kernels for some time after that. However, rest assured that none of this will be done without consideration of the community.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>Given the number of other things in the fully virtualized – paravirtualized spectrum, finding a descriptive name has been difficult. The developers have more or less settled on “PVH” (mainly PV, but with a little bit of HVM), but it has in the past been called other things, including “PV in an HVM container” (or just “HVM containers”), and “Hybrid mode”.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>== What about KVM? ==</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>At this point, some people may be wondering, how would KVM’s virtualization fit into this spectrum?</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>Strictly speaking, KVM is just a set of kernel extensions designed to help processes implement virtualization. When most people speak of using KVM, they mean “qemu-kvm”, which means qemu running configured to use the KVM extensions. (There are other projects, such as the Native Linux KVM tool, which also use the KVM extensions.) When I say “KVM” here, I mean qemu-kvm.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>KVM supports both “legacy boot”, starting in 16-bit mode with a BIOS (or EFI) to load the kernel bootloader, and booting directly into a kernel passed on the qemu command-line. It also provides an emulated motherboard, PCI bus, and so on. It can provide both emulated disk and network cards; and thus it is capable of supporting guests running in fully virtualized mode.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>KVM also provides virtio devices, which can be considered paravirtualized, as well as a PV clock, for operating systems that can be modified to support them. KVM’s typical method of paravirtualization is somewhat different than Xen’s. Virtio devices expose a normal device interface, with MMIO control paths and so on, and could in theory be implemented by real hardware. Xen’s PV interfaces are based on shared memory and lockless synchronization. The kinds of actions that need an MMIO context switch for virtio devices probably correspond pretty closely to actions that need hypercalls for Xen PV devices; but in Xen no instruction emulation needs to be done.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>KVM does not have a paravirtualized interface for timers or interrupts; instead (if I understand correctly) it uses an emulated local APIC. Handling a full interrupt cycle for an emulated local APIC typically requires several MMIO accesses, each of which requires a context switch and an instruction emulation. The Xen PV interrupt interface is based on memory shared with the hypervisor, supplemented by hypercalls when necessary; so most operations can be done without context switches, and those that do require only a single context switch (and no instruction emulation). This was one of the major reasons for introducing PVHVM mode for Xen guests.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>So KVM has paravirtualized devices and a paravirtualized clock, but not paravirtualized interrupts; placing KVM on the spectrum, it would be one step more paravirtualized than “FV with PV drivers”, but not as paravirtualized as PVHVM.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>== The paravirtualization spectrum ==</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>So to summarize: There are a number of things that can be either virtualized or paravirtualized when creating a VM; these include:</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div> Disk and network devices</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div> Interrupts and timers</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div> Emulated platform: motherboard, device buses, BIOS, legacy boot</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div> Privileged instructions and pagetables (memory access)</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>Each of these can be fully virtualized or paravirtualized independently. This leads to a spectrum of virtualization modes, summarized in the table below:</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>http://blog.xen.org/wp-content/uploads/2012/10/pv-spectrum-grid.png</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>The first three of these will all be classified as “HVM mode”, and the last two as “PV mode” for historical reasons. PVH is the new mode, which we expect to be a sweet spot between full virtualization and paravirtualization: it combines the best advantages of Xen’s PV mode with full utilization of hardware support.</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
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<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>Hopefully this has given you an insight into what the various modes are, how they came about, and what are the advantages and disadvantages of each.</div></td>
</tr>
<tr>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>[[Category:Overview]]</div></td>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>[[Category:Overview]]</div></td>
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</table>Lars.kurthhttps://wiki.xenproject.org/index.php?title=Virtualization_Spectrum&diff=5489&oldid=prevLars.kurth at 08:32, 29 October 20122012-10-29T08:32:45Z<p></p>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>At XenSummit 2012 in San Diego, Mukesh Rathor from Oracle presented his work on a [http://vimeo.com/49506288 new virtualization mode], called “PVH”. Adding this mode, there are now a rather dizzying array of different terms thrown about — “HVM”, [[Paravirtualization_(PV)|“PV”]], [[PV_on_HVM|“PVHVM”]], “PVH” — what do they all mean? And why do we have so many?</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>At XenSummit 2012 in San Diego, Mukesh Rathor from Oracle presented his work on a [http://vimeo.com/49506288 new virtualization mode], called “PVH”. Adding this mode, there are now a rather dizzying array of different terms thrown about — “HVM”, [[Paravirtualization_(PV)|“PV”]], [[PV_on_HVM|“PVHVM”]], “PVH” — what do they all mean? And why do we have so many?</div></td>
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</table>Lars.kurthhttps://wiki.xenproject.org/index.php?title=Virtualization_Spectrum&diff=5488&oldid=prevLars.kurth: Added shortcuts2012-10-29T08:32:18Z<p>Added shortcuts</p>
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<td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>| content1 = [[Paravirtualization_(PV)|Paravirtualization]]</div></td>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>At XenSummit 2012 in San Diego, Mukesh Rathor from Oracle presented his work on a [http://vimeo.com/49506288 new virtualization mode], called “PVH”. Adding this mode, there are now a rather dizzying array of different terms thrown about — “HVM”, [[Paravirtualization_(PV)|“PV”]], [[PV_on_HVM|“PVHVM”]], “PVH” — what do they all mean? And why do we have so many?</div></td>
<td class="diff-marker"> </td>
<td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>At XenSummit 2012 in San Diego, Mukesh Rathor from Oracle presented his work on a [http://vimeo.com/49506288 new virtualization mode], called “PVH”. Adding this mode, there are now a rather dizzying array of different terms thrown about — “HVM”, [[Paravirtualization_(PV)|“PV”]], [[PV_on_HVM|“PVHVM”]], “PVH” — what do they all mean? And why do we have so many?</div></td>
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</table>Lars.kurthhttps://wiki.xenproject.org/index.php?title=Virtualization_Spectrum&diff=5487&oldid=prevLars.kurth at 08:27, 29 October 20122012-10-29T08:27:02Z<p></p>
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