| 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). |
