Paravirtualized KVM features

Description

In some cases when implementing hardware interfaces in software is slow, KVM implements its own paravirtualized interfaces.

Setup

Paravirtualized KVM features are represented as CPU flags. The following features are enabled by default for any CPU model when KVM acceleration is enabled:

  • kvmclock

  • kvm-nopiodelay

  • kvm-asyncpf

  • kvm-steal-time

  • kvm-pv-eoi

  • kvmclock-stable-bit

kvm-msi-ext-dest-id feature is enabled by default in x2apic mode with split irqchip (e.g. “-machine …,kernel-irqchip=split -cpu …,x2apic”).

Note: when CPU model host is used, QEMU passes through all supported paravirtualized KVM features to the guest.

Existing features

kvmclock

Expose a KVM specific paravirtualized clocksource to the guest. Supported since Linux v2.6.26.

kvm-nopiodelay

The guest doesn’t need to perform delays on PIO operations. Supported since Linux v2.6.26.

kvm-mmu

This feature is deprecated.

kvm-asyncpf

Enable asynchronous page fault mechanism. Supported since Linux v2.6.38. Note: since Linux v5.10 the feature is deprecated and not enabled by KVM. Use kvm-asyncpf-int instead.

kvm-steal-time

Enable stolen (when guest vCPU is not running) time accounting. Supported since Linux v3.1.

kvm-pv-eoi

Enable paravirtualized end-of-interrupt signaling. Supported since Linux v3.10.

kvm-pv-unhalt

Enable paravirtualized spinlocks support. Supported since Linux v3.12.

kvm-pv-tlb-flush

Enable paravirtualized TLB flush mechanism. Supported since Linux v4.16.

kvm-pv-ipi

Enable paravirtualized IPI mechanism. Supported since Linux v4.19.

kvm-poll-control

Enable host-side polling on HLT control from the guest. Supported since Linux v5.10.

kvm-pv-sched-yield

Enable paravirtualized sched yield feature. Supported since Linux v5.10.

kvm-asyncpf-int

Enable interrupt based asynchronous page fault mechanism. Supported since Linux v5.10.

kvm-msi-ext-dest-id

Support ‘Extended Destination ID’ for external interrupts. The feature allows to use up to 32768 CPUs without IRQ remapping (but other limits may apply making the number of supported vCPUs for a given configuration lower). Supported since Linux v5.10.

kvmclock-stable-bit

Tell the guest that guest visible TSC value can be fully trusted for kvmclock computations and no warps are expected. Supported since Linux v2.6.35.

Supplementary features

kvm-pv-enforce-cpuid

Limit the supported paravirtualized feature set to the exposed features only. Note, by default, KVM allows the guest to use all currently supported paravirtualized features even when they were not announced in guest visible CPUIDs. Supported since Linux v5.10.