Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 20/03/2018 18:35, Vitaly Kuznetsov wrote:
>> +        if (has_msr_hv_frequencies && env->tsc_khz) {
>>              env->features[FEAT_HYPERV_EAX] |= HV_ACCESS_FREQUENCY_MSRS;
>>              env->features[FEAT_HYPERV_EDX] |= HV_FREQUENCY_MSRS_AVAILABLE;
>>          }
>
> Since you have added cpu->hyperv_reenlightenment, I'd rather change this
> so that we don't make the "license to change guest ABI across migration"
> apply more.  We can exploit the fact that Windows doesn't even use the
> MSRs unless either invtsc or re-enlightenment is present.  Something
> like this:
>
>        if (has_msr_hv_frequencies && env->tsc_khz &&
> 	   (tsc_is_stable_and_known(env) ||
>             cpu->hyperv_reenlightenment))
>
> will make the MSRs visible in all useful cases, without having to add
> yet another knob.
>

Can be arranged, of course.

(What I'm worried about with all our hv_* knobs is that more of them we
have easier it is to assemble some frankenstien which won't look like
any existing Hyper-V version; we're probably not doing a very good job
tesing all possible hv_* combinations as people probably use 'all or
nothing'. In case we end up finding a bug in Windows with some weird
hv_* combination it's unlikely Microsoft will bother fixing at as it
doesn't reproduce on any existent Hyper-V version.

That said, it would be great to eventually have something like
'hv_ws2012r2' property making us look exactly the same real WS2012R2
looks like. Unfortunatelly, I'm unsure about a path to get there).

> (Don't worry, this backwards-compatibility stuff is the hardest part.
> I'm so happy that Eduardo is the one maintaining it :)).

I feel the pain :-) Thanks for the reviews!

-- 
  Vitaly