Re: [Qemu-devel] [PATCH v2 1/2] i386/kvm: add support for Hyper-V reenlightenment MSRs

[Paolo Bonzini]

On 19/03/2018 18:06, Roman Kagan wrote:
>> +        if (has_msr_hv_reenlightenment) {
>> +            env->features[FEAT_HYPERV_EAX] |=
>> +                HV_ACCESS_REENLIGHTENMENTS_CONTROL;
>> +        }
> Can you please add a matching comment to the definition of
> feature_word_info[FEAT_HYPERV_EAX].feat_names[]?
> 
> 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