Re: [PATCH] x86/kvm/hyper-v: Don't deactivate APICv unconditionally when Hyper-V SynIC enabled

Paolo Bonzini
2020-11-06T10:18:09Z

On 05/11/20 16:53, Vitaly Kuznetsov wrote:
>> The current implementation of Hyper-V SynIC[1] request to deactivate
>> APICv when SynIC is enabled, since the AutoEOI feature of SynIC is not
>> compatible with APICv[2].
>>
>> Actually, windows doesn't use AutoEOI if deprecating AutoEOI bit is set
>> (CPUID.40000004H:EAX[bit 9], HyperV-TLFS v6.0b section 2.4.5), we don't
>> need to disable APICv in this case.
>>
> Thank you for the patch, the fact that we disable APICv every time we
> enable SynIC is nothing to be proud of. I'm, however, not sure we can
> treat 'Recommend deprecating AutoEOI' as 'AutoEOI must not be
> used.'. Could you please clarify which Windows versions you've tested
> with with?
>

Indeed---older versions of Windows that predate the deprecation will
continue using AutoEOI.

Paolo