| Message ID | 20190329141832.22882-4-vkuznets@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | i386/kvm/hyper-v: refactor and implement 'hv-stimer-direct' and 'hv-all' enlightenments | expand |
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 <vkuznets@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? > + > +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. > + > +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.
Roman Kagan <rkagan@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 <vkuznets@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.
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. + +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. + +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
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> --- docs/hyperv.txt | 180 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 180 insertions(+) create mode 100644 docs/hyperv.txt