Roman Kagan <rka...@virtuozzo.com> writes: > On Fri, Mar 29, 2019 at 03:18:27PM +0100, Vitaly Kuznetsov wrote: >> 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. > > This is appreciated a lot, thanks for doing this! > >> Signed-off-by: Vitaly Kuznetsov <vkuzn...@redhat.com> >> --- >> docs/hyperv.txt | 180 ++++++++++++++++++++++++++++++++++++++++++++++++ >> 1 file changed, 180 insertions(+) >> create mode 100644 docs/hyperv.txt >> >> diff --git a/docs/hyperv.txt b/docs/hyperv.txt >> new file mode 100644 >> index 0000000000..397f2517b8 >> --- /dev/null >> +++ b/docs/hyperv.txt >> @@ -0,0 +1,180 @@ >> +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. > > Hmm, why? >
This was written completely out of top of my head but I was under an impression that writing to HV_X64_MSR_CRASH_CTL causes Qemu to shutdown the guest and Windows does this before it creates crash dump. Am I mistaken? I can be) >> + >> +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 RTC >> +extensively when this enlightenment is not provided; this leads to >> significant >> +CPU consumption, even when virtual CPU is idle. > > I think it'll rather use HPET if available. I'm also not sure the idle > vCPU scenario was one that motivated for implementing paravirtualized > timers. Right but there was an "improvement" from MS in one of their Win10 updates which made things significantly worse so I thought it would be a good idea to document this user-visible property. > >> + >> +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 >> -- >> 2.20.1 >> > > Thanks, > Roman. -- Vitaly