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.
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.
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.
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.
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 notify”.
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).
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).
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.
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.
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)
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.
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.
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).
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.
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.
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.
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.
Note, KVM doesn’t fully support re-enlightenment notifications and doesn’t emulate TSC accesses after migration so ‘tsc-frequency=’ CPU option also has to be specified to make migration succeed. The destination host has to either have the same TSC frequency or support TSC scaling CPU feature.
The enlightenment is nested specific, it targets Hyper-V on KVM guests. When enabled, it provides Enlightened VMCS version 1 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.
Hyper-V specification allows synthetic timer operation in two modes: “classic”, when expiration event is delivered as SynIC message and “direct”, when the event is delivered via normal interrupt. It is known that nested Hyper-V can only use synthetic timers in direct mode and thus
hv-stimer-directneeds to be enabled.
The enlightenment allows to use Hyper-V SynIC with hardware APICv/AVIC enabled. Normally, Hyper-V SynIC disables these hardware feature and suggests the guest to use paravirtualized AutoEOI feature. Note: enabling this feature on old hardware (without APICv/AVIC support) may have negative effect on guest’s performance.
This enlightenment tells guest OS that virtual processors will never share a physical core unless they are reported as sibling SMT threads. This information is required by Windows and Hyper-V guests to properly mitigate SMT related CPU vulnerabilities.
When the option is set to ‘auto’ QEMU will enable the feature only when KVM reports that non-architectural coresharing is impossible, this means that hyper-threading is not supported or completely disabled on the host. This setting also prevents migration as SMT settings on the destination may differ. When the option is set to ‘on’ QEMU will always enable the feature, regardless of host setup. To keep guests secure, this can only be used in conjunction with exposing correct vCPU topology and vCPU pinning.
This changes Hyper-V version identification in CPUID 0x40000002.EAX-EDX from the default (WS2016).
hv-version-id-buildsets ‘Build Number’ (32 bits)
hv-version-id-majorsets ‘Major Version’ (16 bits)
hv-version-id-minorsets ‘Minor Version’ (16 bits)
hv-version-id-spacksets ‘Service Pack’ (32 bits)
hv-version-id-sbranchsets ‘Service Branch’ (8 bits)
hv-version-id-snumbersets ‘Service Number’ (24 bits)
Note: hv-version-id-* are not enlightenments and thus don’t enable Hyper-V identification when specified without any other enlightenments.
Enables Hyper-V synthetic debugger interface, this is a special interface used by Windows Kernel debugger to send the packets through, rather than sending them via serial/network . When enabled, this enlightenment provides additional communication facilities to the guest: SynDbg messages. This new communication is used by Windows Kernel debugger rather than sending packets via serial/network, adding significant performance boost over the other comm channels. This enlightenment requires a VMBus device (-device vmbus-bridge,irq=15).
The enlightenment is nested specific, it targets Hyper-V on KVM guests. When enabled, it allows L0 (KVM) and L1 (Hyper-V) hypervisors to collaborate to avoid unnecessary updates to L2 MSR-Bitmap upon vmexits. While the protocol is supported for both VMX (Intel) and SVM (AMD), the VMX implementation requires Enlightened VMCS (
hv-evmcs) feature to also be enabled.
Hyper-V specification allows to pass parameters for certain hypercalls using XMM registers (“XMM Fast Hypercall Input”). When the feature is in use, it allows for faster hypercalls processing as KVM can avoid reading guest’s memory.
Allow for extended GVA ranges to be passed to Hyper-V TLB flush hypercalls (HvFlushVirtualAddressList/HvFlushVirtualAddressListEx).
The enlightenment is nested specific, it targets Hyper-V on KVM guests. When enabled, it allows L0 (KVM) to directly handle TLB flush hypercalls from L2 guest without the need to exit to L1 (Hyper-V) hypervisor. While the feature is supported for both VMX (Intel) and SVM (AMD), the VMX implementation requires Enlightened VMCS (
hv-evmcs) feature to also be enabled.
In some cases (e.g. during development) it may make sense to use QEMU in ‘pass-through’ mode and give Windows guests all enlightenments currently supported by KVM. This pass-through mode is enabled by “hv-passthrough” CPU flag.
hv-passthroughflag only enables enlightenments which are known to QEMU (have corresponding ‘hv-‘ flag) and copies
hv-vendor-idvalues from KVM to QEMU.
hv-passthroughoverrides all other ‘hv-‘ settings on the command line. Also, enabling this flag effectively prevents migration as the list of enabled enlightenments may differ between target and destination hosts.
By default, KVM allows the guest to use all currently supported Hyper-V enlightenments when Hyper-V CPUID interface was exposed, regardless of if some features were not announced in guest visible CPUIDs.
hv-enforce-cpuidfeature alters this behavior and only allows the guest to use exposed Hyper-V enlightenments.