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diff --git a/docs/hyperv.txt b/docs/hyperv.txt new file mode 100644 index 0000000000..c423e0fca2 --- /dev/null +++ b/docs/hyperv.txt @@ -0,0 +1,181 @@ +Hyper-V Enlightenments +====================== + + +1. Description +=============== +In some cases when implementing a hardware interface in software is slow, KVM +implements its own paravirtualized interfaces. This works well for Linux as +guest support for such features is added simultaneously with the feature itself. +It may, however, be hard-to-impossible to add support for these interfaces to +proprietary OSes, namely, Microsoft Windows. + +KVM on x86 implements Hyper-V Enlightenments for Windows guests. These features +make Windows and Hyper-V guests think they're running on top of a Hyper-V +compatible hypervisor and use Hyper-V specific features. + + +2. Setup +========= +No Hyper-V enlightenments are enabled by default by either KVM or QEMU. In +QEMU, individual enlightenments can be enabled through CPU flags, e.g: + + qemu-system-x86_64 --enable-kvm --cpu host,hv_relaxed,hv_vpindex,hv_time, ... + +Sometimes there are dependencies between enlightenments, QEMU is supposed to +check that the supplied configuration is sane. + +When any set of the Hyper-V enlightenments is enabled, QEMU changes hypervisor +identification (CPUID 0x40000000..0x4000000A) to Hyper-V. KVM identification +and features are kept in leaves 0x40000100..0x40000101. + + +3. Existing enlightenments +=========================== + +3.1. hv-relaxed +================ +This feature tells guest OS to disable watchdog timeouts as it is running on a +hypervisor. It is known that some Windows versions will do this even when they +see 'hypervisor' CPU flag. + +3.2. hv-vapic +============== +Provides so-called VP Assist page MSR to guest allowing it to work with APIC +more efficiently. In particular, this enlightenment allows paravirtualized +(exit-less) EOI processing. + +3.3. hv-spinlocks=xxx +====================== +Enables paravirtualized spinlocks. The parameter indicates how many times +spinlock acquisition should be attempted before indicating the situation to the +hypervisor. A special value 0xffffffff indicates "never to retry". + +3.4. hv-vpindex +================ +Provides HV_X64_MSR_VP_INDEX (0x40000002) MSR to the guest which has Virtual +processor index information. This enlightenment makes sense in conjunction with +hv-synic, hv-stimer and other enlightenments which require the guest to know its +Virtual Processor indices (e.g. when VP index needs to be passed in a +hypercall). + +3.5. hv-runtime +================ +Provides HV_X64_MSR_VP_RUNTIME (0x40000010) MSR to the guest. The MSR keeps the +virtual processor run time in 100ns units. This gives guest operating system an +idea of how much time was 'stolen' from it (when the virtual CPU was preempted +to perform some other work). + +3.6. hv-crash +============== +Provides HV_X64_MSR_CRASH_P0..HV_X64_MSR_CRASH_P5 (0x40000100..0x40000105) and +HV_X64_MSR_CRASH_CTL (0x40000105) MSRs to the guest. These MSRs are written to +by the guest when it crashes, HV_X64_MSR_CRASH_P0..HV_X64_MSR_CRASH_P5 MSRs +contain additional crash information. This information is outputted in QEMU log +and through QAPI. +Note: unlike under genuine Hyper-V, write to HV_X64_MSR_CRASH_CTL causes guest +to shutdown. This effectively blocks crash dump generation by Windows. + +3.7. hv-time +============= +Enables two Hyper-V-specific clocksources available to the guest: MSR-based +Hyper-V clocksource (HV_X64_MSR_TIME_REF_COUNT, 0x40000020) and Reference TSC +page (enabled via MSR HV_X64_MSR_REFERENCE_TSC, 0x40000021). Both clocksources +are per-guest, Reference TSC page clocksource allows for exit-less time stamp +readings. Using this enlightenment leads to significant speedup of all timestamp +related operations. + +3.8. hv-synic +============== +Enables Hyper-V Synthetic interrupt controller - an extension of a local APIC. +When enabled, this enlightenment provides additional communication facilities +to the guest: SynIC messages and Events. This is a pre-requisite for +implementing VMBus devices (not yet in QEMU). Additionally, this enlightenment +is needed to enable Hyper-V synthetic timers. SynIC is controlled through MSRs +HV_X64_MSR_SCONTROL..HV_X64_MSR_EOM (0x40000080..0x40000084) and +HV_X64_MSR_SINT0..HV_X64_MSR_SINT15 (0x40000090..0x4000009F) + +Requires: hv-vpindex + +3.9. hv-stimer +=============== +Enables Hyper-V synthetic timers. There are four synthetic timers per virtual +CPU controlled through HV_X64_MSR_STIMER0_CONFIG..HV_X64_MSR_STIMER3_COUNT +(0x400000B0..0x400000B7) MSRs. These timers can work either in single-shot or +periodic mode. It is known that certain Windows versions revert to using HPET +(or even RTC when HPET is unavailable) extensively when this enlightenment is +not provided; this can lead to significant CPU consumption, even when virtual +CPU is idle. + +Requires: hv-vpindex, hv-synic, hv-time + +3.10. hv-tlbflush +================== +Enables paravirtualized TLB shoot-down mechanism. On x86 architecture, remote +TLB flush procedure requires sending IPIs and waiting for other CPUs to perform +local TLB flush. In virtualized environment some virtual CPUs may not even be +scheduled at the time of the call and may not require flushing (or, flushing +may be postponed until the virtual CPU is scheduled). hv-tlbflush enlightenment +implements TLB shoot-down through hypervisor enabling the optimization. + +Requires: hv-vpindex + +3.11. hv-ipi +============= +Enables paravirtualized IPI send mechanism. HvCallSendSyntheticClusterIpi +hypercall may target more than 64 virtual CPUs simultaneously, doing the same +through APIC requires more than one access (and thus exit to the hypervisor). + +Requires: hv-vpindex + +3.12. hv-vendor-id=xxx +======================= +This changes Hyper-V identification in CPUID 0x40000000.EBX-EDX from the default +"Microsoft Hv". The parameter should be no longer than 12 characters. According +to the specification, guests shouldn't use this information and it is unknown +if there is a Windows version which acts differently. +Note: hv-vendor-id is not an enlightenment and thus doesn't enable Hyper-V +identification when specified without some other enlightenment. + +3.13. hv-reset +=============== +Provides HV_X64_MSR_RESET (0x40000003) MSR to the guest allowing it to reset +itself by writing to it. Even when this MSR is enabled, it is not a recommended +way for Windows to perform system reboot and thus it may not be used. + +3.14. hv-frequencies +============================================ +Provides HV_X64_MSR_TSC_FREQUENCY (0x40000022) and HV_X64_MSR_APIC_FREQUENCY +(0x40000023) allowing the guest to get its TSC/APIC frequencies without doing +measurements. + +3.15 hv-reenlightenment +======================== +The enlightenment is nested specific, it targets Hyper-V on KVM guests. When +enabled, it provides HV_X64_MSR_REENLIGHTENMENT_CONTROL (0x40000106), +HV_X64_MSR_TSC_EMULATION_CONTROL (0x40000107)and HV_X64_MSR_TSC_EMULATION_STATUS +(0x40000108) MSRs allowing the guest to get notified when TSC frequency changes +(only happens on migration) and keep using old frequency (through emulation in +the hypervisor) until it is ready to switch to the new one. This, in conjunction +with hv-frequencies, allows Hyper-V on KVM to pass stable clocksource (Reference +TSC page) to its own guests. + +Recommended: hv-frequencies + +3.16. hv-evmcs +=============== +The enlightenment is nested specific, it targets Hyper-V on KVM guests. When +enabled, it provides Enlightened VMCS feature to the guest. The feature +implements paravirtualized protocol between L0 (KVM) and L1 (Hyper-V) +hypervisors making L2 exits to the hypervisor faster. The feature is Intel-only. +Note: some virtualization features (e.g. Posted Interrupts) are disabled when +hv-evmcs is enabled. It may make sense to measure your nested workload with and +without the feature to find out if enabling it is beneficial. + +Requires: hv-vapic + + +4. Useful links +================ +Hyper-V Top Level Functional specification and other information: +https://github.com/MicrosoftDocs/Virtualization-Documentation |