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docs: Update Xen-on-KVM documentation for PV disk support

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Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
This commit is contained in:
David Woodhouse 2023-03-07 10:54:31 +00:00
parent 438bec498b
commit 154eac3719
1 changed files with 23 additions and 7 deletions
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30
docs/system/i386/xen.rst
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@ -9,6 +9,8 @@ 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
-----
@ -17,14 +19,14 @@ accelerator, for example for Xen 4.10:
.. parsed-literal::
|qemu_system| --accel kvm,xen-version=0x4000a
|qemu_system| --accel kvm,xen-version=0x4000a,kernel-irqchip=split
Additionally, virtual APIC support can be advertised to the guest through the
``xen-vapic`` CPU flag:
.. parsed-literal::
|qemu_system| --accel kvm,xen-version=0x4000a --cpu host,+xen_vapic
|qemu_system| --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
@ -33,11 +35,25 @@ 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. Note that until the Xen PV device back ends are enabled to work
with Xen mode in QEMU, that is unlikely to cause significant joy. Linux guests
can be dissuaded from this by adding 'xen_emul_unplug=never' on their command
line, and it can also be noted that AHCI disk controllers are exempt from being
unplugged, as are passthrough VFIO PCI devices.
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
.. parsed-literal::
|qemu_system| -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
----------