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Re: [Qemu-devel] [PATCH 2/2] i386/kvm: lower requirements for Hyper-V fr
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure |
Date: |
Fri, 16 Mar 2018 17:04:08 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 |
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.
[Qemu-devel] [PATCH 1/2] i386/kvm: add support for Hyper-V reenlightenment MSRs, Vitaly Kuznetsov, 2018/03/12