PVH (v2) Domu

Starting from Xen Project 4.8, the Xen Project team has been working on overhauling the x86 core of the Xen Project Hypervisor. One major work item in progress is PVHv2 (Dom0 and DomU) and splitting the PV and HVM code paths, to enable building a HVM and PVHv2 only hypervisor.

PVHv2 DomU

Xen 4.10 adds a PVHv2 xl/libxl stable interface for unprivileged guests. Although PVHv2 DomU support has been present in previous Xen releases the lack of a stable toolstack interface prevented users from reliably testing and deploying this feature. From Xen 4.10 onwards PVHv2 DomU is a supported feature.

PVHv2 guests are lightweight HVM guests which use PV drivers for I/O and native interfaces for the rest of the operations. Unlike HVM guests, PVHv2 guests do use QEMU device emulation. This reduces the memory footprint of Xen Project based systems significantly as for each HVM guest a QEMU instance runs in Dom0.

In addition, PVHv2 relies on Hardware Virtualization extensions and does neither use the PV kernel infrastructure nor the PV MMU, significantly reducing the number of Xen specific interfaces a PVHv2 guest uses compared to PV.

Consequently, PVHv2 guests have a much smaller TCB and attack surface compared to PV and HVM guests. Removing a large component such as QEMU, which consists of approximately 1.2 million lines of code - twice as much as the Xen Project Hypervisor itself, significantly reduces the potential of security vulnerabilities in a Xen Project based software stack compared to HVM guests.

In contrast to HVM based virtualization, PVHv2 does require operating system support, which is available in Linux 4.11 or newer.

PVH History

In Xen Project 4.4 and 4.5 we introduced a virtualization mode called PVH: This is essentially a PV guest using PV drivers for boot and I/O and hardware virtualization extensions for everything else. In late 2015, we started an initiative to re-architect and simplify PVH: PVHv2 was born. PVHv2 addresses key limitations of PVHv1, such as: not restricted to a specific paging mode decided at boot time, less usage of hypercalls and availability of some emulated platform devices provided by Xen itself.

Xen Project 4.8 laid the groundwork for PVHv2. In Xen Project 4.9 we completed most of the Hypervisor portion of PVHv2 and removed PVHv1. In Xen Project 4.10 we completed PVHv2 DomU by providing a stable interface in the toolstack in order to manage PVHv2 guests. We also delivered all necessary Linux Functionality for PVHv2 Guest support in Linux 4.11.

What is next?

PVHv2 support for FreeBSD is currently being reviewed, but has not yet been committed. Support for Dom0 PVHv2 and pci-passthrough for DomU will follow in a subsequent release. Work on supporting EFI boot in addition to support for Direct Kernel Boot is in progress.

In addition, we started the groundwork for wrapping the PV ABI inside a PVH container, which will eventually allow removal of the PV ABI from Xen and the Linux kernel, while allowing users to run legacy PV guest images on hardware with virtualization extension.