1Xen HVM guest support 2===================== 3 4 5Description 6----------- 7 8KVM has support for hosting Xen guests, intercepting Xen hypercalls and event 9channel (Xen PV interrupt) delivery. This allows guests which expect to be 10run under Xen to be hosted in QEMU under Linux/KVM instead. 11 12Setup 13----- 14 15Xen mode is enabled by setting the ``xen-version`` property of the KVM 16accelerator, for example for Xen 4.10: 17 18.. parsed-literal:: 19 20 |qemu_system| --accel kvm,xen-version=0x4000a 21 22Additionally, virtual APIC support can be advertised to the guest through the 23``xen-vapic`` CPU flag: 24 25.. parsed-literal:: 26 27 |qemu_system| --accel kvm,xen-version=0x4000a --cpu host,+xen_vapic 28 29When Xen support is enabled, QEMU changes hypervisor identification (CPUID 300x40000000..0x4000000A) to Xen. The KVM identification and features are not 31advertised to a Xen guest. If Hyper-V is also enabled, the Xen identification 32moves to leaves 0x40000100..0x4000010A. 33 34The Xen platform device is enabled automatically for a Xen guest. This allows 35a guest to unplug all emulated devices, in order to use Xen PV block and network 36drivers instead. Note that until the Xen PV device back ends are enabled to work 37with Xen mode in QEMU, that is unlikely to cause significant joy. Linux guests 38can be dissuaded from this by adding 'xen_emul_unplug=never' on their command 39line, and it can also be noted that AHCI disk controllers are exempt from being 40unplugged, as are passthrough VFIO PCI devices. 41 42Properties 43---------- 44 45The following properties exist on the KVM accelerator object: 46 47``xen-version`` 48 This property contains the Xen version in ``XENVER_version`` form, with the 49 major version in the top 16 bits and the minor version in the low 16 bits. 50 Setting this property enables the Xen guest support. 51 52``xen-evtchn-max-pirq`` 53 Xen PIRQs represent an emulated physical interrupt, either GSI or MSI, which 54 can be routed to an event channel instead of to the emulated I/O or local 55 APIC. By default, QEMU permits only 256 PIRQs because this allows maximum 56 compatibility with 32-bit MSI where the higher bits of the PIRQ# would need 57 to be in the upper 64 bits of the MSI message. For guests with large numbers 58 of PCI devices (and none which are limited to 32-bit addressing) it may be 59 desirable to increase this value. 60 61``xen-gnttab-max-frames`` 62 Xen grant tables are the means by which a Xen guest grants access to its 63 memory for PV back ends (disk, network, etc.). Since QEMU only supports v1 64 grant tables which are 8 bytes in size, each page (each frame) of the grant 65 table can reference 512 pages of guest memory. The default number of frames 66 is 64, allowing for 32768 pages of guest memory to be accessed by PV backends 67 through simultaneous grants. For guests with large numbers of PV devices and 68 high throughput, it may be desirable to increase this value. 69 70OS requirements 71--------------- 72 73The minimal Xen support in the KVM accelerator requires the host to be running 74Linux v5.12 or newer. Later versions add optimisations: Linux v5.17 added 75acceleration of interrupt delivery via the Xen PIRQ mechanism, and Linux v5.19 76accelerated Xen PV timers and inter-processor interrupts (IPIs). 77