Re: [PATCH 0/3] KVM: x86: Hyper-V invariant TSC control feature

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Maxim Levitsky <mlevitsk@redhat.com> writes:

> On Wed, 2022-07-13 at 17:05 +0200, Vitaly Kuznetsov wrote:
>> Normally, genuine Hyper-V doesn't expose architectural invariant TSC
>> (CPUID.80000007H:EDX[8]) to its guests by default. A special PV MSR
>> (HV_X64_MSR_TSC_INVARIANT_CONTROL, 0x40000118) and corresponding CPUID
>> feature bit (CPUID.0x40000003.EAX[15]) were introduced. When bit 0 of the
>> PV MSR is set, invariant TSC bit starts to show up in CPUID. When the 
>> feature is exposed to Hyper-V guests, reenlightenment becomes unneeded.
>
> If I understood the feature correctly from the code, it allows the HyperV, or in this
> case KVM acting as HyperV, to avoid unconditionally exposing the invltsc bit
> in CPUID, but rather let the guest know that it can opt-in into this,
> by giving the guest another CPUID bit to indicate this ability
> and a MSR which the guest uses to opt-in.
>
> Are there known use cases of this, are there guests which won't opt-in?
>

Linux prior to dce7cd62754b and some older Windows guests I guess.

>> 
>> Note: strictly speaking, KVM doesn't have to have the feature as exposing
>> raw invariant TSC bit (CPUID.80000007H:EDX[8]) also seems to work for
>> modern Windows versions. The feature is, however, tiny and straitforward
>> and gives additional flexibility so why not.
>
> This means that KVM can also just unconditionally expose the invtsc bit
> to the guest, and the guest still uses it.

Yes, this feature doesn't bring much by itself (at least with modern
Windows versions). I've implemented it while debugging what ended up
being 
https://lore.kernel.org/kvm/20220712135009.952805-1-vkuznets@redhat.com/
(so the issue wasn't enlightenments related after all) but as I think it
may come handy some day so why keeping it in my private stash.

>
>
> Nitpick: It might be worth it to document it a bit better somewhere,
> as I tried to do in this mail.

TLFS sounds like the right place for it but ... it's not there... oh well.

>
>
> Best regards,
> 	Maxim Levitsky
>
>> 
>> Vitaly Kuznetsov (3):
>>   KVM: x86: Hyper-V invariant TSC control
>>   KVM: selftests: Fix wrmsr_safe()
>>   KVM: selftests: Test Hyper-V invariant TSC control
>> 
>>  arch/x86/include/asm/kvm_host.h               |  1 +
>>  arch/x86/kvm/cpuid.c                          |  7 ++
>>  arch/x86/kvm/hyperv.c                         | 19 +++++
>>  arch/x86/kvm/hyperv.h                         | 15 ++++
>>  arch/x86/kvm/x86.c                            |  4 +-
>>  .../selftests/kvm/include/x86_64/processor.h  |  2 +-
>>  .../selftests/kvm/x86_64/hyperv_features.c    | 73 ++++++++++++++++++-
>>  7 files changed, 115 insertions(+), 6 deletions(-)
>> 
>
>

-- 
Vitaly