i386/kvm: document existing Hyper-V enlightenments
Currently, there is no doc describing hv-* CPU flags, people are encouraged to get the information from Microsoft Hyper-V Top Level Functional specification (TLFS). There is, however, a bit of QEMU specifics. Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com> Message-Id: <20190517141924.19024-5-vkuznets@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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Vitaly Kuznetsov
Paolo Bonzini
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docs/hyperv.txt
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docs/hyperv.txt
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Hyper-V Enlightenments
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======================
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1. Description
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===============
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In some cases when implementing a hardware interface in software is slow, KVM
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implements its own paravirtualized interfaces. This works well for Linux as
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guest support for such features is added simultaneously with the feature itself.
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It may, however, be hard-to-impossible to add support for these interfaces to
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proprietary OSes, namely, Microsoft Windows.
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KVM on x86 implements Hyper-V Enlightenments for Windows guests. These features
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make Windows and Hyper-V guests think they're running on top of a Hyper-V
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compatible hypervisor and use Hyper-V specific features.
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2. Setup
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=========
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No Hyper-V enlightenments are enabled by default by either KVM or QEMU. In
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QEMU, individual enlightenments can be enabled through CPU flags, e.g:
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qemu-system-x86_64 --enable-kvm --cpu host,hv_relaxed,hv_vpindex,hv_time, ...
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Sometimes there are dependencies between enlightenments, QEMU is supposed to
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check that the supplied configuration is sane.
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When any set of the Hyper-V enlightenments is enabled, QEMU changes hypervisor
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identification (CPUID 0x40000000..0x4000000A) to Hyper-V. KVM identification
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and features are kept in leaves 0x40000100..0x40000101.
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3. Existing enlightenments
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===========================
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3.1. hv-relaxed
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================
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This feature tells guest OS to disable watchdog timeouts as it is running on a
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hypervisor. It is known that some Windows versions will do this even when they
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see 'hypervisor' CPU flag.
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3.2. hv-vapic
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==============
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Provides so-called VP Assist page MSR to guest allowing it to work with APIC
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more efficiently. In particular, this enlightenment allows paravirtualized
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(exit-less) EOI processing.
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3.3. hv-spinlocks=xxx
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======================
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Enables paravirtualized spinlocks. The parameter indicates how many times
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spinlock acquisition should be attempted before indicating the situation to the
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hypervisor. A special value 0xffffffff indicates "never to retry".
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3.4. hv-vpindex
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================
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Provides HV_X64_MSR_VP_INDEX (0x40000002) MSR to the guest which has Virtual
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processor index information. This enlightenment makes sense in conjunction with
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hv-synic, hv-stimer and other enlightenments which require the guest to know its
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Virtual Processor indices (e.g. when VP index needs to be passed in a
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hypercall).
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3.5. hv-runtime
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================
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Provides HV_X64_MSR_VP_RUNTIME (0x40000010) MSR to the guest. The MSR keeps the
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virtual processor run time in 100ns units. This gives guest operating system an
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idea of how much time was 'stolen' from it (when the virtual CPU was preempted
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to perform some other work).
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3.6. hv-crash
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==============
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Provides HV_X64_MSR_CRASH_P0..HV_X64_MSR_CRASH_P5 (0x40000100..0x40000105) and
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HV_X64_MSR_CRASH_CTL (0x40000105) MSRs to the guest. These MSRs are written to
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by the guest when it crashes, HV_X64_MSR_CRASH_P0..HV_X64_MSR_CRASH_P5 MSRs
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contain additional crash information. This information is outputted in QEMU log
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and through QAPI.
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Note: unlike under genuine Hyper-V, write to HV_X64_MSR_CRASH_CTL causes guest
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to shutdown. This effectively blocks crash dump generation by Windows.
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3.7. hv-time
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=============
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Enables two Hyper-V-specific clocksources available to the guest: MSR-based
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Hyper-V clocksource (HV_X64_MSR_TIME_REF_COUNT, 0x40000020) and Reference TSC
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page (enabled via MSR HV_X64_MSR_REFERENCE_TSC, 0x40000021). Both clocksources
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are per-guest, Reference TSC page clocksource allows for exit-less time stamp
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readings. Using this enlightenment leads to significant speedup of all timestamp
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related operations.
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3.8. hv-synic
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==============
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Enables Hyper-V Synthetic interrupt controller - an extension of a local APIC.
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When enabled, this enlightenment provides additional communication facilities
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to the guest: SynIC messages and Events. This is a pre-requisite for
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implementing VMBus devices (not yet in QEMU). Additionally, this enlightenment
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is needed to enable Hyper-V synthetic timers. SynIC is controlled through MSRs
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HV_X64_MSR_SCONTROL..HV_X64_MSR_EOM (0x40000080..0x40000084) and
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HV_X64_MSR_SINT0..HV_X64_MSR_SINT15 (0x40000090..0x4000009F)
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Requires: hv-vpindex
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3.9. hv-stimer
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===============
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Enables Hyper-V synthetic timers. There are four synthetic timers per virtual
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CPU controlled through HV_X64_MSR_STIMER0_CONFIG..HV_X64_MSR_STIMER3_COUNT
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(0x400000B0..0x400000B7) MSRs. These timers can work either in single-shot or
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periodic mode. It is known that certain Windows versions revert to using HPET
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(or even RTC when HPET is unavailable) extensively when this enlightenment is
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not provided; this can lead to significant CPU consumption, even when virtual
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CPU is idle.
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Requires: hv-vpindex, hv-synic, hv-time
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3.10. hv-tlbflush
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==================
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Enables paravirtualized TLB shoot-down mechanism. On x86 architecture, remote
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TLB flush procedure requires sending IPIs and waiting for other CPUs to perform
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local TLB flush. In virtualized environment some virtual CPUs may not even be
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scheduled at the time of the call and may not require flushing (or, flushing
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may be postponed until the virtual CPU is scheduled). hv-tlbflush enlightenment
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implements TLB shoot-down through hypervisor enabling the optimization.
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Requires: hv-vpindex
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3.11. hv-ipi
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=============
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Enables paravirtualized IPI send mechanism. HvCallSendSyntheticClusterIpi
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hypercall may target more than 64 virtual CPUs simultaneously, doing the same
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through APIC requires more than one access (and thus exit to the hypervisor).
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Requires: hv-vpindex
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3.12. hv-vendor-id=xxx
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=======================
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This changes Hyper-V identification in CPUID 0x40000000.EBX-EDX from the default
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"Microsoft Hv". The parameter should be no longer than 12 characters. According
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to the specification, guests shouldn't use this information and it is unknown
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if there is a Windows version which acts differently.
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Note: hv-vendor-id is not an enlightenment and thus doesn't enable Hyper-V
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identification when specified without some other enlightenment.
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3.13. hv-reset
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===============
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Provides HV_X64_MSR_RESET (0x40000003) MSR to the guest allowing it to reset
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itself by writing to it. Even when this MSR is enabled, it is not a recommended
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way for Windows to perform system reboot and thus it may not be used.
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3.14. hv-frequencies
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============================================
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Provides HV_X64_MSR_TSC_FREQUENCY (0x40000022) and HV_X64_MSR_APIC_FREQUENCY
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(0x40000023) allowing the guest to get its TSC/APIC frequencies without doing
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measurements.
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3.15 hv-reenlightenment
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========================
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The enlightenment is nested specific, it targets Hyper-V on KVM guests. When
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enabled, it provides HV_X64_MSR_REENLIGHTENMENT_CONTROL (0x40000106),
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HV_X64_MSR_TSC_EMULATION_CONTROL (0x40000107)and HV_X64_MSR_TSC_EMULATION_STATUS
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(0x40000108) MSRs allowing the guest to get notified when TSC frequency changes
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(only happens on migration) and keep using old frequency (through emulation in
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the hypervisor) until it is ready to switch to the new one. This, in conjunction
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with hv-frequencies, allows Hyper-V on KVM to pass stable clocksource (Reference
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TSC page) to its own guests.
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Recommended: hv-frequencies
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3.16. hv-evmcs
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===============
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The enlightenment is nested specific, it targets Hyper-V on KVM guests. When
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enabled, it provides Enlightened VMCS feature to the guest. The feature
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implements paravirtualized protocol between L0 (KVM) and L1 (Hyper-V)
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hypervisors making L2 exits to the hypervisor faster. The feature is Intel-only.
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Note: some virtualization features (e.g. Posted Interrupts) are disabled when
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hv-evmcs is enabled. It may make sense to measure your nested workload with and
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without the feature to find out if enabling it is beneficial.
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Requires: hv-vapic
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4. Useful links
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================
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Hyper-V Top Level Functional specification and other information:
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https://github.com/MicrosoftDocs/Virtualization-Documentation
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