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 12Using the split irqchip is mandatory for Xen support. 13 14Setup 15----- 16 17Xen mode is enabled by setting the ``xen-version`` property of the KVM 18accelerator, for example for Xen 4.10: 19 20.. parsed-literal:: 21 22 |qemu_system| --accel kvm,xen-version=0x4000a,kernel-irqchip=split 23 24Additionally, virtual APIC support can be advertised to the guest through the 25``xen-vapic`` CPU flag: 26 27.. parsed-literal:: 28 29 |qemu_system| --accel kvm,xen-version=0x4000a,kernel-irqchip=split --cpu host,+xen_vapic 30 31When Xen support is enabled, QEMU changes hypervisor identification (CPUID 320x40000000..0x4000000A) to Xen. The KVM identification and features are not 33advertised to a Xen guest. If Hyper-V is also enabled, the Xen identification 34moves to leaves 0x40000100..0x4000010A. 35 36The Xen platform device is enabled automatically for a Xen guest. This allows 37a guest to unplug all emulated devices, in order to use Xen PV block and network 38drivers instead. Under Xen, the boot disk is typically available both via IDE 39emulation, and as a PV block device. Guest bootloaders typically use IDE to load 40the guest kernel, which then unplugs the IDE and continues with the Xen PV block 41device. 42 43This configuration can be achieved as follows 44 45.. parsed-literal:: 46 47 |qemu_system| -M pc --accel kvm,xen-version=0x4000a,kernel-irqchip=split \\ 48 -drive file=${GUEST_IMAGE},if=none,id=disk,file.locking=off -device xen-disk,drive=disk,vdev=xvda \\ 49 -drive file=${GUEST_IMAGE},index=2,media=disk,file.locking=off,if=ide 50 51It is necessary to use the pc machine type, as the q35 machine uses AHCI instead 52of legacy IDE, and AHCI disks are not unplugged through the Xen PV unplug 53mechanism. 54 55VirtIO devices can also be used; Linux guests may need to be dissuaded from 56umplugging them by adding 'xen_emul_unplug=never' on their command line. 57 58Properties 59---------- 60 61The following properties exist on the KVM accelerator object: 62 63``xen-version`` 64 This property contains the Xen version in ``XENVER_version`` form, with the 65 major version in the top 16 bits and the minor version in the low 16 bits. 66 Setting this property enables the Xen guest support. 67 68``xen-evtchn-max-pirq`` 69 Xen PIRQs represent an emulated physical interrupt, either GSI or MSI, which 70 can be routed to an event channel instead of to the emulated I/O or local 71 APIC. By default, QEMU permits only 256 PIRQs because this allows maximum 72 compatibility with 32-bit MSI where the higher bits of the PIRQ# would need 73 to be in the upper 64 bits of the MSI message. For guests with large numbers 74 of PCI devices (and none which are limited to 32-bit addressing) it may be 75 desirable to increase this value. 76 77``xen-gnttab-max-frames`` 78 Xen grant tables are the means by which a Xen guest grants access to its 79 memory for PV back ends (disk, network, etc.). Since QEMU only supports v1 80 grant tables which are 8 bytes in size, each page (each frame) of the grant 81 table can reference 512 pages of guest memory. The default number of frames 82 is 64, allowing for 32768 pages of guest memory to be accessed by PV backends 83 through simultaneous grants. For guests with large numbers of PV devices and 84 high throughput, it may be desirable to increase this value. 85 86OS requirements 87--------------- 88 89The minimal Xen support in the KVM accelerator requires the host to be running 90Linux v5.12 or newer. Later versions add optimisations: Linux v5.17 added 91acceleration of interrupt delivery via the Xen PIRQ mechanism, and Linux v5.19 92accelerated Xen PV timers and inter-processor interrupts (IPIs). 93