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From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <b4974d25-de6f-7a88-4416-faa6764a76cc@redhat.com>
Date: Tue, 20 Mar 2018 13:00:09 +0100
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Subject: Re: [Qemu-devel] [PATCH v2 1/2] i386/kvm: add support for Hyper-V
 reenlightenment MSRs
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To: Roman Kagan <rkagan@virtuozzo.com>, Vitaly Kuznetsov <vkuznets@redhat.com>, Richard Henderson <rth@twiddle.net>, Eduardo Habkost <ehabkost@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>, qemu-devel@nongnu.org

On 19/03/2018 18:06, Roman Kagan wrote:
>> +        if (has_msr_hv_reenlightenment) {
>> +            env->features[FEAT_HYPERV_EAX] |=3D
>> +                HV_ACCESS_REENLIGHTENMENTS_CONTROL;
>> +        }
> Can you please add a matching comment to the definition of
> feature_word_info[FEAT_HYPERV_EAX].feat_names[]?
>=20
> Also there appears to be no cpu property to turn this on/off, does it?
> It's enabled based only on the support in the KVM it's running against.
> So I guess we may have a problem migrating between the hosts with
> different KVM versions, one supporting it and the other not.
> (This is also a problem with has_msr_hv_frequencies, and is in general =
a
> long-standing issue of hv_* properties being done differently from the
> rest of CPUID features.)

Yeah, so far for frequencies that was okay or at least okay-ish because
Windows didn't use it by default without invtsc (disables migration).

However, we do need a new hv_reenlight property; if it is not set, QEMU
should behave as if the reenlightenment MSRs are not provided by KVM.

Thanks,

Paolo