1Paravirtualized KVM features 2============================ 3 4Description 5----------- 6 7In some cases when implementing hardware interfaces in software is slow, ``KVM`` 8implements its own paravirtualized interfaces. 9 10Setup 11----- 12 13Paravirtualized ``KVM`` features are represented as CPU flags. The following 14features are enabled by default for any CPU model when ``KVM`` acceleration is 15enabled: 16 17- ``kvmclock`` 18- ``kvm-nopiodelay`` 19- ``kvm-asyncpf`` 20- ``kvm-steal-time`` 21- ``kvm-pv-eoi`` 22- ``kvmclock-stable-bit`` 23 24``kvm-msi-ext-dest-id`` feature is enabled by default in x2apic mode with split 25irqchip (e.g. "-machine ...,kernel-irqchip=split -cpu ...,x2apic"). 26 27Note: when CPU model ``host`` is used, QEMU passes through all supported 28paravirtualized ``KVM`` features to the guest. 29 30Existing features 31----------------- 32 33``kvmclock`` 34 Expose a ``KVM`` specific paravirtualized clocksource to the guest. Supported 35 since Linux v2.6.26. 36 37``kvm-nopiodelay`` 38 The guest doesn't need to perform delays on PIO operations. Supported since 39 Linux v2.6.26. 40 41``kvm-mmu`` 42 This feature is deprecated. 43 44``kvm-asyncpf`` 45 Enable asynchronous page fault mechanism. Supported since Linux v2.6.38. 46 Note: since Linux v5.10 the feature is deprecated and not enabled by ``KVM``. 47 Use ``kvm-asyncpf-int`` instead. 48 49``kvm-steal-time`` 50 Enable stolen (when guest vCPU is not running) time accounting. Supported 51 since Linux v3.1. 52 53``kvm-pv-eoi`` 54 Enable paravirtualized end-of-interrupt signaling. Supported since Linux 55 v3.10. 56 57``kvm-pv-unhalt`` 58 Enable paravirtualized spinlocks support. Supported since Linux v3.12. 59 60``kvm-pv-tlb-flush`` 61 Enable paravirtualized TLB flush mechanism. Supported since Linux v4.16. 62 63``kvm-pv-ipi`` 64 Enable paravirtualized IPI mechanism. Supported since Linux v4.19. 65 66``kvm-poll-control`` 67 Enable host-side polling on HLT control from the guest. Supported since Linux 68 v5.10. 69 70``kvm-pv-sched-yield`` 71 Enable paravirtualized sched yield feature. Supported since Linux v5.10. 72 73``kvm-asyncpf-int`` 74 Enable interrupt based asynchronous page fault mechanism. Supported since Linux 75 v5.10. 76 77``kvm-msi-ext-dest-id`` 78 Support 'Extended Destination ID' for external interrupts. The feature allows 79 to use up to 32768 CPUs without IRQ remapping (but other limits may apply making 80 the number of supported vCPUs for a given configuration lower). Supported since 81 Linux v5.10. 82 83``kvmclock-stable-bit`` 84 Tell the guest that guest visible TSC value can be fully trusted for kvmclock 85 computations and no warps are expected. Supported since Linux v2.6.35. 86 87Supplementary features 88---------------------- 89 90``kvm-pv-enforce-cpuid`` 91 Limit the supported paravirtualized feature set to the exposed features only. 92 Note, by default, ``KVM`` allows the guest to use all currently supported 93 paravirtualized features even when they were not announced in guest visible 94 CPUIDs. Supported since Linux v5.10. 95 96 97Useful links 98------------ 99 100Please refer to Documentation/virt/kvm in Linux for additional details. 101