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From: Vitaly Kuznetsov <vkuznets@redhat.com>
In-Reply-To: <c30c335e-ca21-1179-670b-d4506ce14571@redhat.com>
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	<c30c335e-ca21-1179-670b-d4506ce14571@redhat.com>
Date: Thu, 29 Nov 2018 12:51:55 +0100
Message-ID: <878t1chzj8.fsf@vitty.brq.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] i386/kvm: expose
 HV_CPUID_ENLIGHTMENT_INFO.EAX and HV_CPUID_NESTED_FEATURES.EAX as feature
 words
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To: Paolo Bonzini <pbonzini@redhat.com>, qemu-devel@nongnu.org
Cc: Richard Henderson <rth@twiddle.net>, Eduardo Habkost <ehabkost@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>, Roman Kagan <rkagan@virtuozzo.com>

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 26/11/18 14:59, Vitaly Kuznetsov wrote:
>> It was found that QMP users of QEMU (e.g. libvirt) may need
>> HV_CPUID_ENLIGHTMENT_INFO.EAX/HV_CPUID_NESTED_FEATURES.EAX information. In
>> particular, 'hv_tlbflush' and 'hv_evmcs' enlightenments are only exposed in
>> HV_CPUID_ENLIGHTMENT_INFO.EAX.
>> 
>> HV_CPUID_NESTED_FEATURES.EAX is exposed for two reasons: convenience
>> (we don't need to export it from hyperv_handle_properties() and as
>> future-proof for Enlightened MSR-Bitmap, PV EPT invalidation and
>> direct virtual flush features.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>
> Can you add a comment to feature_word_info, explaining why the
> feat_names are not set?

I had to do some code archeology to make sure I understand, I think it
goes back to

http://lists.gnu.org/archive/html/qemu-devel/2016-06/msg06579.html

So the comment (probably added before FEAT_HYPERV_EAX definition) would
be

".feat_names are commented out for Hyper-V enlightenments because we
don't want to have two different ways for enabling them on QEMU command
line. Some features (e.g. "hyperv_time", "hyperv_vapic", ...) require
enabling several feature bits simultaneously, exposing these bits
individually may just confuse guests."

Would do?

-- 
Vitaly