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[v9,53/58] i386/xen: Document Xen HVM emulation

Message ID 20230128081113.1615111-54-dwmw2@infradead.org (mailing list archive)
State New, archived
Headers show
Series Xen HVM support under KVM | expand

Commit Message

David Woodhouse Jan. 28, 2023, 8:11 a.m. UTC
From: David Woodhouse <dwmw@amazon.co.uk>

Signed-off-by: David Woodhouse <dwmw2@amazon.co.uk>
 docs/system/i386/xen.rst    | 50 +++++++++++++++++++++++++++++++++++++
 docs/system/target-i386.rst |  1 +
 2 files changed, 51 insertions(+)
 create mode 100644 docs/system/i386/xen.rst
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diff --git a/docs/system/i386/xen.rst b/docs/system/i386/xen.rst
new file mode 100644
index 0000000000..71506eb5c4
--- /dev/null
+++ b/docs/system/i386/xen.rst
@@ -0,0 +1,50 @@ 
+Xen HVM guest support
+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.
+Xen mode is enabled by setting the ``xen-version`` property of the KVM
+accelerator to a 32-bit value in the ``XENVER_version`` form, with the Xen
+major version in the top 16 bits and the minor version in the low 16 bits,
+for example for Xen 4.10:
+.. parsed-literal::
+  |qemu_system| --accel kvm,xen-version=0x4000a
+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
+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. 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.
+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).
diff --git a/docs/system/target-i386.rst b/docs/system/target-i386.rst
index e64c013077..77c2f3b979 100644
--- a/docs/system/target-i386.rst
+++ b/docs/system/target-i386.rst
@@ -27,6 +27,7 @@  Architectural features
+   i386/xen