i386/xen: Document Xen HVM emulation

Signed-off-by: David Woodhouse <dwmw2@amazon.co.uk>

diff --git a/docs/system/i386/xen.rst b/docs/system/i386/xen.rst
new file mode 100644 (file)
index 0000000..71506eb
--- /dev/null
+++ b/docs/system/i386/xen.rst
@@ -0,0 +1,50 @@
+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.
+
+Setup
+-----
+
+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 e64c0130772d1b63559089b96d48b1e2421c0ed0..77c2f3b979992f1296d21606c66a02b88d5886ac 100644 (file)
--- a/docs/system/target-i386.rst
+++ b/docs/system/target-i386.rst
@@ -27,6 +27,7 @@ Architectural features
 
    i386/cpu
    i386/hyperv
+   i386/xen
    i386/kvm-pv
    i386/sgx
    i386/amd-memory-encryption