Xen PVOPS CPU microcode update support
Xen has support for updating the CPU microcode on Linux through two mechanisms, the early boot time CPU microcode patching mechanism and live run time updating. The preferred and recommended mechanism is to do use the early boot time CPU microcode patching unless there are mechanisms in place to ensure updates to the CPU microcode get propagated to userspace properly. At times there may also be reasons why microcode cannot be updated at runtime, one example would be the HSW disabling patch for AMD which can only be installed late if userspace has not used TSX instructions yet. Cases that require systems to not reboot for long periods of time may receive special attention and care to ensure live boot updates are possible but special care must be taken to ensure this will room smoothly. What each distribution chooses to support varies.
Early boot time CPU microcode patching
Early boot time CPU microcode patching got initial support via the Xen 4.2 release with the addition of the 'ucode=<digit>' boot loader command line parameter which can be passed on to Xen which enabled support for specific microcode files to be used from a list of specified grub modules. Refer to the Xen 4.2 command line documentation for more details. Xen was later enhanced through the 4.5 release with 'ucode=scan' support which makes use of the new Linux early microcode format, for details refer to the Xen 4.5 command line documentation, Xen commit 155587481e392e4261038364e2761aab27f597ed, and Linux documentation Documentation/x86/early-microcode.txt.
Run time microcode updates
Run time updates can be supported by using the xenmicrocode (never merged upstream) tool. Some distributions have historically used a sysfs rescan mechanism to enable automatic run time updates but this is known to only be enabled now on Debian only. Distributions wishing to use run time updates of microcode on Xen should consider making modifying the kernel so that rescan request return 0 and does nothing when Xen is detected. It is advisable to not return an error on Xen given that errors on udev can be chatty and users may end up opening bug reports for it. In order to make use of run time microcode updates userspace for a distribution can be updated to use the proposed xenmicrocode tool instead when Xen is detected. Care must be taken to ensure that only certain microcode files are used given that some are known to not work well via live updates.
Debian is known to handle run time updates on Debian stable by whitelisting known well working microcode files. Debian unstable does not allow any run time updates as black listing is a reactive process. Distributions should take care to only enable live updates of CPU microcode on special circumstances.