214 lines
9.3 KiB
Text
214 lines
9.3 KiB
Text
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
|
|
|
|
3.17. hv-stimer-direct
|
|
=======================
|
|
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-direct' needs to be
|
|
enabled.
|
|
|
|
Requires: hv-vpindex, hv-synic, hv-time, hv-stimer
|
|
|
|
3.17. hv-no-nonarch-coresharing=on/off/auto
|
|
===========================================
|
|
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.
|
|
|
|
4. Development features
|
|
========================
|
|
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.
|
|
Note: enabling this flag effectively prevents migration as supported features
|
|
may differ between target and destination.
|
|
|
|
|
|
4. Useful links
|
|
================
|
|
Hyper-V Top Level Functional specification and other information:
|
|
https://github.com/MicrosoftDocs/Virtualization-Documentation
|