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154eac3719
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk> Reviewed-by: Paul Durrant <paul@xen.org>
93 lines
3.7 KiB
ReStructuredText
93 lines
3.7 KiB
ReStructuredText
Xen HVM guest support
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=====================
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Description
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-----------
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KVM has support for hosting Xen guests, intercepting Xen hypercalls and event
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channel (Xen PV interrupt) delivery. This allows guests which expect to be
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run under Xen to be hosted in QEMU under Linux/KVM instead.
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Using the split irqchip is mandatory for Xen support.
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Setup
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-----
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Xen mode is enabled by setting the ``xen-version`` property of the KVM
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accelerator, for example for Xen 4.10:
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.. parsed-literal::
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|qemu_system| --accel kvm,xen-version=0x4000a,kernel-irqchip=split
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Additionally, virtual APIC support can be advertised to the guest through the
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``xen-vapic`` CPU flag:
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.. parsed-literal::
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|qemu_system| --accel kvm,xen-version=0x4000a,kernel-irqchip=split --cpu host,+xen_vapic
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When Xen support is enabled, QEMU changes hypervisor identification (CPUID
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0x40000000..0x4000000A) to Xen. The KVM identification and features are not
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advertised to a Xen guest. If Hyper-V is also enabled, the Xen identification
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moves to leaves 0x40000100..0x4000010A.
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The Xen platform device is enabled automatically for a Xen guest. This allows
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a guest to unplug all emulated devices, in order to use Xen PV block and network
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drivers instead. Under Xen, the boot disk is typically available both via IDE
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emulation, and as a PV block device. Guest bootloaders typically use IDE to load
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the guest kernel, which then unplugs the IDE and continues with the Xen PV block
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device.
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This configuration can be achieved as follows
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.. parsed-literal::
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|qemu_system| -M pc --accel kvm,xen-version=0x4000a,kernel-irqchip=split \\
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-drive file=${GUEST_IMAGE},if=none,id=disk,file.locking=off -device xen-disk,drive=disk,vdev=xvda \\
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-drive file=${GUEST_IMAGE},index=2,media=disk,file.locking=off,if=ide
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It is necessary to use the pc machine type, as the q35 machine uses AHCI instead
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of legacy IDE, and AHCI disks are not unplugged through the Xen PV unplug
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mechanism.
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VirtIO devices can also be used; Linux guests may need to be dissuaded from
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umplugging them by adding 'xen_emul_unplug=never' on their command line.
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Properties
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----------
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The following properties exist on the KVM accelerator object:
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``xen-version``
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This property contains the Xen version in ``XENVER_version`` form, with the
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major version in the top 16 bits and the minor version in the low 16 bits.
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Setting this property enables the Xen guest support.
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``xen-evtchn-max-pirq``
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Xen PIRQs represent an emulated physical interrupt, either GSI or MSI, which
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can be routed to an event channel instead of to the emulated I/O or local
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APIC. By default, QEMU permits only 256 PIRQs because this allows maximum
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compatibility with 32-bit MSI where the higher bits of the PIRQ# would need
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to be in the upper 64 bits of the MSI message. For guests with large numbers
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of PCI devices (and none which are limited to 32-bit addressing) it may be
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desirable to increase this value.
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``xen-gnttab-max-frames``
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Xen grant tables are the means by which a Xen guest grants access to its
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memory for PV back ends (disk, network, etc.). Since QEMU only supports v1
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grant tables which are 8 bytes in size, each page (each frame) of the grant
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table can reference 512 pages of guest memory. The default number of frames
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is 64, allowing for 32768 pages of guest memory to be accessed by PV backends
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through simultaneous grants. For guests with large numbers of PV devices and
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high throughput, it may be desirable to increase this value.
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OS requirements
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---------------
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The minimal Xen support in the KVM accelerator requires the host to be running
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Linux v5.12 or newer. Later versions add optimisations: Linux v5.17 added
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acceleration of interrupt delivery via the Xen PIRQ mechanism, and Linux v5.19
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accelerated Xen PV timers and inter-processor interrupts (IPIs).
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