Xen HVM guest support

Description

KVM has support for hosting Xen guests, intercepting Xen hypercalls and event channel (Xen PV interrupt) delivery. This allows guests which expect to be run under Xen to be hosted in QEMU under Linux/KVM instead.

Using the split irqchip is mandatory for Xen support.

Setup

Xen mode is enabled by setting the xen-version property of the KVM accelerator, for example for Xen 4.10:

qemu-system-x86_64 --accel kvm,xen-version=0x4000a,kernel-irqchip=split

Additionally, virtual APIC support can be advertised to the guest through the xen-vapic CPU flag:

qemu-system-x86_64 --accel kvm,xen-version=0x4000a,kernel-irqchip=split --cpu host,+xen_vapic

When Xen support is enabled, QEMU changes hypervisor identification (CPUID 0x40000000..0x4000000A) to Xen. The KVM identification and features are not advertised to a Xen guest. If Hyper-V is also enabled, the Xen identification moves to leaves 0x40000100..0x4000010A.

The Xen platform device is enabled automatically for a Xen guest. This allows a guest to unplug all emulated devices, in order to use Xen PV block and network drivers instead. Under Xen, the boot disk is typically available both via IDE emulation, and as a PV block device. Guest bootloaders typically use IDE to load the guest kernel, which then unplugs the IDE and continues with the Xen PV block device.

This configuration can be achieved as follows

qemu-system-x86_64 -M pc --accel kvm,xen-version=0x4000a,kernel-irqchip=split \
     -drive file=${GUEST_IMAGE},if=none,id=disk,file.locking=off -device xen-disk,drive=disk,vdev=xvda \
     -drive file=${GUEST_IMAGE},index=2,media=disk,file.locking=off,if=ide

It is necessary to use the pc machine type, as the q35 machine uses AHCI instead of legacy IDE, and AHCI disks are not unplugged through the Xen PV unplug mechanism.

VirtIO devices can also be used; Linux guests may need to be dissuaded from umplugging them by adding ‘xen_emul_unplug=never’ on their command line.

Properties

The following properties exist on the KVM accelerator object:

xen-version: This property contains the Xen version in XENVER_version form, with the major version in the top 16 bits and the minor version in the low 16 bits. Setting this property enables the Xen guest support.
xen-evtchn-max-pirq: Xen PIRQs represent an emulated physical interrupt, either GSI or MSI, which can be routed to an event channel instead of to the emulated I/O or local APIC. By default, QEMU permits only 256 PIRQs because this allows maximum compatibility with 32-bit MSI where the higher bits of the PIRQ# would need to be in the upper 64 bits of the MSI message. For guests with large numbers of PCI devices (and none which are limited to 32-bit addressing) it may be desirable to increase this value.
xen-gnttab-max-frames: Xen grant tables are the means by which a Xen guest grants access to its memory for PV back ends (disk, network, etc.). Since QEMU only supports v1 grant tables which are 8 bytes in size, each page (each frame) of the grant table can reference 512 pages of guest memory. The default number of frames is 64, allowing for 32768 pages of guest memory to be accessed by PV backends through simultaneous grants. For guests with large numbers of PV devices and high throughput, it may be desirable to increase this value.

OS requirements

The minimal Xen support in the KVM accelerator requires the host to be running Linux v5.12 or newer. Later versions add optimisations: Linux v5.17 added acceleration of interrupt delivery via the Xen PIRQ mechanism, and Linux v5.19 accelerated Xen PV timers and inter-processor interrupts (IPIs).