Re: [PATCH] KVM: hyperv: support HV_X64_MSR_TSC_FREQUENCY and HV_X64_MSR_APIC_FREQUENCY

["Radim Krčmář" <rkrcmar@redhat.com>]

2017-07-26 11:47+0200, Ladi Prosek:
> On Mon, Jul 24, 2017 at 4:23 PM, Radim Krčmář <rkrcmar@redhat.com> wrote:
> > 2017-07-17 15:28+0200, Ladi Prosek:
> >> The newly supported MSRs are advertised with the AccessFrequencyRegs
> >> partition privilege flag and CPUID.40000003H:EDX[8] "Support for
> >> determining timer frequencies is available" (both outside of the scope of
> >> this KVM patch).
> >
> > Do Windows ignore the AccessFrequencyMsrs (CPUID.40000003H:EAX[11]) bit?
> 
> No, Windows won't read the MSRs without CPUID.40000003H:EAX[11]. The
> other bit (CPUID.40000003H:EDX[8]) appears to be ignored, or at least
> I couldn't find any difference in behavior.

Looking more into it, we are emulating non-root partitions, where
CPUID.40000003H:EDX[8] would probably be clear and then the check Linux
has in ms_hyperv_init_platform() could be wrong.  I am not sure what
FrequencyMsrsAvailable is for, though -- to allow AccessFrequencyMsrs in
non-root even if root has AccessFrequencyMsrs disabled?

> I wonder why you're asking, i.e. what is it that I'm misunderstanding :)

I was just curious why you didn't mention both in the same way
(CamelCase name or CPUID bit) and the best explanation was that you
didn't need to care about one, because Windows worked without it. :)
I think you got it all right.

Thanks.