From c1eaa6d0df6ed9e021f751d0be6eb321551a9bea Mon Sep 17 00:00:00 2001 From: David Woodhouse Date: Tue, 17 Jan 2023 14:13:32 +0000 Subject: i386/xen: Document Xen HVM emulation Signed-off-by: David Woodhouse Reviewed-by: Paul Durrant --- docs/system/i386/xen.rst | 76 +++++++++++++++++++++++++++++++++++++++++++++ docs/system/target-i386.rst | 1 + 2 files changed, 77 insertions(+) create mode 100644 docs/system/i386/xen.rst (limited to 'docs') diff --git a/docs/system/i386/xen.rst b/docs/system/i386/xen.rst new file mode 100644 index 0000000..a00523b --- /dev/null +++ b/docs/system/i386/xen.rst @@ -0,0 +1,76 @@ +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, 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. + +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). diff --git a/docs/system/target-i386.rst b/docs/system/target-i386.rst index e64c013..77c2f3b 100644 --- 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 -- cgit v1.1