101 lines
3.0 KiB
ReStructuredText
101 lines
3.0 KiB
ReStructuredText
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Paravirtualized KVM features
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============================
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Description
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-----------
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In some cases when implementing hardware interfaces in software is slow, ``KVM``
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implements its own paravirtualized interfaces.
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Setup
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-----
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Paravirtualized ``KVM`` features are represented as CPU flags. The following
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features are enabled by default for any CPU model when ``KVM`` acceleration is
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enabled:
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- ``kvmclock``
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- ``kvm-nopiodelay``
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- ``kvm-asyncpf``
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- ``kvm-steal-time``
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- ``kvm-pv-eoi``
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- ``kvmclock-stable-bit``
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``kvm-msi-ext-dest-id`` feature is enabled by default in x2apic mode with split
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irqchip (e.g. "-machine ...,kernel-irqchip=split -cpu ...,x2apic").
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Note: when CPU model ``host`` is used, QEMU passes through all supported
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paravirtualized ``KVM`` features to the guest.
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Existing features
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-----------------
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``kvmclock``
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Expose a ``KVM`` specific paravirtualized clocksource to the guest. Supported
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since Linux v2.6.26.
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``kvm-nopiodelay``
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The guest doesn't need to perform delays on PIO operations. Supported since
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Linux v2.6.26.
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``kvm-mmu``
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This feature is deprecated.
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``kvm-asyncpf``
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Enable asynchronous page fault mechanism. Supported since Linux v2.6.38.
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Note: since Linux v5.10 the feature is deprecated and not enabled by ``KVM``.
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Use ``kvm-asyncpf-int`` instead.
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``kvm-steal-time``
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Enable stolen (when guest vCPU is not running) time accounting. Supported
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since Linux v3.1.
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``kvm-pv-eoi``
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Enable paravirtualized end-of-interrupt signaling. Supported since Linux
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v3.10.
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``kvm-pv-unhalt``
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Enable paravirtualized spinlocks support. Supported since Linux v3.12.
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``kvm-pv-tlb-flush``
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Enable paravirtualized TLB flush mechanism. Supported since Linux v4.16.
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``kvm-pv-ipi``
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Enable paravirtualized IPI mechanism. Supported since Linux v4.19.
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``kvm-poll-control``
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Enable host-side polling on HLT control from the guest. Supported since Linux
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v5.10.
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``kvm-pv-sched-yield``
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Enable paravirtualized sched yield feature. Supported since Linux v5.10.
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``kvm-asyncpf-int``
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Enable interrupt based asynchronous page fault mechanism. Supported since Linux
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v5.10.
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``kvm-msi-ext-dest-id``
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Support 'Extended Destination ID' for external interrupts. The feature allows
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to use up to 32768 CPUs without IRQ remapping (but other limits may apply making
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the number of supported vCPUs for a given configuration lower). Supported since
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Linux v5.10.
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``kvmclock-stable-bit``
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Tell the guest that guest visible TSC value can be fully trusted for kvmclock
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computations and no warps are expected. Supported since Linux v2.6.35.
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Supplementary features
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----------------------
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``kvm-pv-enforce-cpuid``
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Limit the supported paravirtualized feature set to the exposed features only.
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Note, by default, ``KVM`` allows the guest to use all currently supported
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paravirtualized features even when they were not announced in guest visible
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CPUIDs. Supported since Linux v5.10.
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Useful links
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------------
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Please refer to Documentation/virt/kvm in Linux for additional details.
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