RE: [PATCH 1/2] x86/hyperv: Allow guests to enable InvariantTSC

[Vitaly Kuznetsov]

Michael Kelley <mikelley@xxxxxxxxxxxxx> writes:

> From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> Sent: Friday, October 4, 2019 9:57 AM
>> 
>> Andrea Parri <parri.andrea@xxxxxxxxx> writes:
>> 
>> > If the hardware supports TSC scaling, Hyper-V will set bit 15 of the
>> > HV_PARTITION_PRIVILEGE_MASK in guest VMs with a compatible Hyper-V
>> > configuration version.  Bit 15 corresponds to the
>> > AccessTscInvariantControls privilege.  If this privilege bit is set,
>> > guests can access the HvSyntheticInvariantTscControl MSR: guests can
>> > set bit 0 of this synthetic MSR to enable the InvariantTSC feature.
>> > After setting the synthetic MSR, CPUID will enumerate support for
>> > InvariantTSC.
>> 
>> I tried getting more information from TLFS but as of 5.0C this feature
>> is not described there. I'm really interested in why this additional
>> interface is needed, e.g. why can't Hyper-V just set InvariantTSC
>> unconditionally when TSC scaling is supported?
>> 
>
> Yes, this is very new functionality that is not yet available in a released
> version of Hyper-V.  And as you know, the Hyper-V TLFS has gotten
> woefully out-of-date. :-(
>
> Your question is the same question I asked.   The reason given by
> Hyper-V is to take the more cautious approach of not "automatically"
> giving VMs an InvariantTSC due to updating the underlying Hyper-V
> version.  Instead, guest VMs must have been explicitly coded to take
> advantage of the new InvariantTSC feature.  It's not clear to me how
> much of this caution is driven by Windows guests vs. Linux or FreeBSD
> guests, but it is what it is.
>
> Having to explicitly enable the InvariantTSC does give the Linux code
> the opportunity to be a bit cleaner by doing things like not marking
> the TSC as unstable when the InvariantTSC feature is present, and to
> mark the TSC as reliable so we don't try to do TSC synchronization
> (which Hyper-V does not want guests to try to do).

Thank you for these additional details Michael,

we'll probably have to add support for this bit to KVM and I'd like to
know the background. From Linux perspective, no matter what's the
interface we'd like to get InvariantTSC.

Feel free to add

Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

-- 
Vitaly