| 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. |
| |
| |
| Useful links |
| ------------ |
| |
| Please refer to Documentation/virt/kvm in Linux for additional details. |
