qemu/docs/hyperv.txt
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   1Hyper-V Enlightenments
   2======================
   3
   4
   51. Description
   6===============
   7In some cases when implementing a hardware interface in software is slow, KVM
   8implements its own paravirtualized interfaces. This works well for Linux as
   9guest support for such features is added simultaneously with the feature itself.
  10It may, however, be hard-to-impossible to add support for these interfaces to
  11proprietary OSes, namely, Microsoft Windows.
  12
  13KVM on x86 implements Hyper-V Enlightenments for Windows guests. These features
  14make Windows and Hyper-V guests think they're running on top of a Hyper-V
  15compatible hypervisor and use Hyper-V specific features.
  16
  17
  182. Setup
  19=========
  20No Hyper-V enlightenments are enabled by default by either KVM or QEMU. In
  21QEMU, individual enlightenments can be enabled through CPU flags, e.g:
  22
  23  qemu-system-x86_64 --enable-kvm --cpu host,hv_relaxed,hv_vpindex,hv_time, ...
  24
  25Sometimes there are dependencies between enlightenments, QEMU is supposed to
  26check that the supplied configuration is sane.
  27
  28When any set of the Hyper-V enlightenments is enabled, QEMU changes hypervisor
  29identification (CPUID 0x40000000..0x4000000A) to Hyper-V. KVM identification
  30and features are kept in leaves 0x40000100..0x40000101.
  31
  32
  333. Existing enlightenments
  34===========================
  35
  363.1. hv-relaxed
  37================
  38This feature tells guest OS to disable watchdog timeouts as it is running on a
  39hypervisor. It is known that some Windows versions will do this even when they
  40see 'hypervisor' CPU flag.
  41
  423.2. hv-vapic
  43==============
  44Provides so-called VP Assist page MSR to guest allowing it to work with APIC
  45more efficiently. In particular, this enlightenment allows paravirtualized
  46(exit-less) EOI processing.
  47
  483.3. hv-spinlocks=xxx
  49======================
  50Enables paravirtualized spinlocks. The parameter indicates how many times
  51spinlock acquisition should be attempted before indicating the situation to the
  52hypervisor. A special value 0xffffffff indicates "never notify".
  53
  543.4. hv-vpindex
  55================
  56Provides HV_X64_MSR_VP_INDEX (0x40000002) MSR to the guest which has Virtual
  57processor index information. This enlightenment makes sense in conjunction with
  58hv-synic, hv-stimer and other enlightenments which require the guest to know its
  59Virtual Processor indices (e.g. when VP index needs to be passed in a
  60hypercall).
  61
  623.5. hv-runtime
  63================
  64Provides HV_X64_MSR_VP_RUNTIME (0x40000010) MSR to the guest. The MSR keeps the
  65virtual processor run time in 100ns units. This gives guest operating system an
  66idea of how much time was 'stolen' from it (when the virtual CPU was preempted
  67to perform some other work).
  68
  693.6. hv-crash
  70==============
  71Provides HV_X64_MSR_CRASH_P0..HV_X64_MSR_CRASH_P5 (0x40000100..0x40000105) and
  72HV_X64_MSR_CRASH_CTL (0x40000105) MSRs to the guest. These MSRs are written to
  73by the guest when it crashes, HV_X64_MSR_CRASH_P0..HV_X64_MSR_CRASH_P5 MSRs
  74contain additional crash information. This information is outputted in QEMU log
  75and through QAPI.
  76Note: unlike under genuine Hyper-V, write to HV_X64_MSR_CRASH_CTL causes guest
  77to shutdown. This effectively blocks crash dump generation by Windows.
  78
  793.7. hv-time
  80=============
  81Enables two Hyper-V-specific clocksources available to the guest: MSR-based
  82Hyper-V clocksource (HV_X64_MSR_TIME_REF_COUNT, 0x40000020) and Reference TSC
  83page (enabled via MSR HV_X64_MSR_REFERENCE_TSC, 0x40000021). Both clocksources
  84are per-guest, Reference TSC page clocksource allows for exit-less time stamp
  85readings. Using this enlightenment leads to significant speedup of all timestamp
  86related operations.
  87
  883.8. hv-synic
  89==============
  90Enables Hyper-V Synthetic interrupt controller - an extension of a local APIC.
  91When enabled, this enlightenment provides additional communication facilities
  92to the guest: SynIC messages and Events. This is a pre-requisite for
  93implementing VMBus devices (not yet in QEMU). Additionally, this enlightenment
  94is needed to enable Hyper-V synthetic timers. SynIC is controlled through MSRs
  95HV_X64_MSR_SCONTROL..HV_X64_MSR_EOM (0x40000080..0x40000084) and
  96HV_X64_MSR_SINT0..HV_X64_MSR_SINT15 (0x40000090..0x4000009F)
  97
  98Requires: hv-vpindex
  99
 1003.9. hv-stimer
 101===============
 102Enables Hyper-V synthetic timers. There are four synthetic timers per virtual
 103CPU controlled through HV_X64_MSR_STIMER0_CONFIG..HV_X64_MSR_STIMER3_COUNT
 104(0x400000B0..0x400000B7) MSRs. These timers can work either in single-shot or
 105periodic mode. It is known that certain Windows versions revert to using HPET
 106(or even RTC when HPET is unavailable) extensively when this enlightenment is
 107not provided; this can lead to significant CPU consumption, even when virtual
 108CPU is idle.
 109
 110Requires: hv-vpindex, hv-synic, hv-time
 111
 1123.10. hv-tlbflush
 113==================
 114Enables paravirtualized TLB shoot-down mechanism. On x86 architecture, remote
 115TLB flush procedure requires sending IPIs and waiting for other CPUs to perform
 116local TLB flush. In virtualized environment some virtual CPUs may not even be
 117scheduled at the time of the call and may not require flushing (or, flushing
 118may be postponed until the virtual CPU is scheduled). hv-tlbflush enlightenment
 119implements TLB shoot-down through hypervisor enabling the optimization.
 120
 121Requires: hv-vpindex
 122
 1233.11. hv-ipi
 124=============
 125Enables paravirtualized IPI send mechanism. HvCallSendSyntheticClusterIpi
 126hypercall may target more than 64 virtual CPUs simultaneously, doing the same
 127through APIC requires more than one access (and thus exit to the hypervisor).
 128
 129Requires: hv-vpindex
 130
 1313.12. hv-vendor-id=xxx
 132=======================
 133This changes Hyper-V identification in CPUID 0x40000000.EBX-EDX from the default
 134"Microsoft Hv". The parameter should be no longer than 12 characters. According
 135to the specification, guests shouldn't use this information and it is unknown
 136if there is a Windows version which acts differently.
 137Note: hv-vendor-id is not an enlightenment and thus doesn't enable Hyper-V
 138identification when specified without some other enlightenment.
 139
 1403.13. hv-reset
 141===============
 142Provides HV_X64_MSR_RESET (0x40000003) MSR to the guest allowing it to reset
 143itself by writing to it. Even when this MSR is enabled, it is not a recommended
 144way for Windows to perform system reboot and thus it may not be used.
 145
 1463.14. hv-frequencies
 147============================================
 148Provides HV_X64_MSR_TSC_FREQUENCY (0x40000022) and HV_X64_MSR_APIC_FREQUENCY
 149(0x40000023) allowing the guest to get its TSC/APIC frequencies without doing
 150measurements.
 151
 1523.15 hv-reenlightenment
 153========================
 154The enlightenment is nested specific, it targets Hyper-V on KVM guests. When
 155enabled, it provides HV_X64_MSR_REENLIGHTENMENT_CONTROL (0x40000106),
 156HV_X64_MSR_TSC_EMULATION_CONTROL (0x40000107)and HV_X64_MSR_TSC_EMULATION_STATUS
 157(0x40000108) MSRs allowing the guest to get notified when TSC frequency changes
 158(only happens on migration) and keep using old frequency (through emulation in
 159the hypervisor) until it is ready to switch to the new one. This, in conjunction
 160with hv-frequencies, allows Hyper-V on KVM to pass stable clocksource (Reference
 161TSC page) to its own guests.
 162
 163Note, KVM doesn't fully support re-enlightenment notifications and doesn't
 164emulate TSC accesses after migration so 'tsc-frequency=' CPU option also has to
 165be specified to make migration succeed. The destination host has to either have
 166the same TSC frequency or support TSC scaling CPU feature.
 167
 168Recommended: hv-frequencies
 169
 1703.16. hv-evmcs
 171===============
 172The enlightenment is nested specific, it targets Hyper-V on KVM guests. When
 173enabled, it provides Enlightened VMCS version 1 feature to the guest. The feature
 174implements paravirtualized protocol between L0 (KVM) and L1 (Hyper-V)
 175hypervisors making L2 exits to the hypervisor faster. The feature is Intel-only.
 176Note: some virtualization features (e.g. Posted Interrupts) are disabled when
 177hv-evmcs is enabled. It may make sense to measure your nested workload with and
 178without the feature to find out if enabling it is beneficial.
 179
 180Requires: hv-vapic
 181
 1823.17. hv-stimer-direct
 183=======================
 184Hyper-V specification allows synthetic timer operation in two modes: "classic",
 185when expiration event is delivered as SynIC message and "direct", when the event
 186is delivered via normal interrupt. It is known that nested Hyper-V can only
 187use synthetic timers in direct mode and thus 'hv-stimer-direct' needs to be
 188enabled.
 189
 190Requires: hv-vpindex, hv-synic, hv-time, hv-stimer
 191
 1923.18. hv-avic (hv-apicv)
 193=======================
 194The enlightenment allows to use Hyper-V SynIC with hardware APICv/AVIC enabled.
 195Normally, Hyper-V SynIC disables these hardware feature and suggests the guest
 196to use paravirtualized AutoEOI feature.
 197Note: enabling this feature on old hardware (without APICv/AVIC support) may
 198have negative effect on guest's performance.
 199
 2003.19. hv-no-nonarch-coresharing=on/off/auto
 201===========================================
 202This enlightenment tells guest OS that virtual processors will never share a
 203physical core unless they are reported as sibling SMT threads. This information
 204is required by Windows and Hyper-V guests to properly mitigate SMT related CPU
 205vulnerabilities.
 206When the option is set to 'auto' QEMU will enable the feature only when KVM
 207reports that non-architectural coresharing is impossible, this means that
 208hyper-threading is not supported or completely disabled on the host. This
 209setting also prevents migration as SMT settings on the destination may differ.
 210When the option is set to 'on' QEMU will always enable the feature, regardless
 211of host setup. To keep guests secure, this can only be used in conjunction with
 212exposing correct vCPU topology and vCPU pinning.
 213
 2143.20. hv-version-id-{build,major,minor,spack,sbranch,snumber}
 215=============================================================
 216This changes Hyper-V version identification in CPUID 0x40000002.EAX-EDX from the
 217default (WS2016).
 218- hv-version-id-build sets 'Build Number' (32 bits)
 219- hv-version-id-major sets 'Major Version' (16 bits)
 220- hv-version-id-minor sets 'Minor Version' (16 bits)
 221- hv-version-id-spack sets 'Service Pack' (32 bits)
 222- hv-version-id-sbranch sets 'Service Branch' (8 bits)
 223- hv-version-id-snumber sets 'Service Number' (24 bits)
 224
 225Note: hv-version-id-* are not enlightenments and thus don't enable Hyper-V
 226identification when specified without any other enlightenments.
 227
 2284. Supplementary features
 229=========================
 230
 2314.1. hv-passthrough
 232===================
 233In some cases (e.g. during development) it may make sense to use QEMU in
 234'pass-through' mode and give Windows guests all enlightenments currently
 235supported by KVM. This pass-through mode is enabled by "hv-passthrough" CPU
 236flag.
 237Note: "hv-passthrough" flag only enables enlightenments which are known to QEMU
 238(have corresponding "hv-*" flag) and copies "hv-spinlocks="/"hv-vendor-id="
 239values from KVM to QEMU. "hv-passthrough" overrides all other "hv-*" settings on
 240the command line. Also, enabling this flag effectively prevents migration as the
 241list of enabled enlightenments may differ between target and destination hosts.
 242
 2434.2. hv-enforce-cpuid
 244=====================
 245By default, KVM allows the guest to use all currently supported Hyper-V
 246enlightenments when Hyper-V CPUID interface was exposed, regardless of if
 247some features were not announced in guest visible CPUIDs. 'hv-enforce-cpuid'
 248feature alters this behavior and only allows the guest to use exposed Hyper-V
 249enlightenments.
 250
 251
 2525. Useful links
 253================
 254Hyper-V Top Level Functional specification and other information:
 255https://github.com/MicrosoftDocs/Virtualization-Documentation
 256