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

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/03/2018 16:40, Vitaly Kuznetsov wrote:
>> On older kernel without re-enlightenment support, you don't want to
>> expose the frequency MSRs unless invtsc is on, right?
>>
> Actually no, I think it's OK to expose frequency MSRs even when we don't
> have invtsc and don't support re-enlightenment. Nested Hyper-V won't
> pass stable TSC pages to its guests unless it sees either invtsc or
> reenlightenment. So as long as we have something to put to these MSRs
> (env->tsc_khz) I *think* we can expose them.
> 
> I may actually be missing the reason why Ladi put
> tsc_is_stable_and_known() here.

Probably because I asked him to. :)  It looks like Hyper-V knows that
you need re-enlightenment in order to really trust the frequency MSRs
(of course the TSC page is special because it has the sequence count).

So the patch is good.  Thanks!

Paolo

> In case we're running Windows (and not
> Hyper-V) as a guest KVM will update TSC page on migration. And genuine
> Hyper-V also exposes these MSRs without exposing INVTSC flag by
> default.