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

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Date:   Fri, 6 Nov 2020 11:18:01 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, junjiehua0xff@...il.com
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Andrey Smetanin <asmetanin@...tuozzo.com>,
        Junjie Hua <junjiehua@...cent.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/kvm/hyper-v: Don't deactivate APICv unconditionally
 when Hyper-V SynIC enabled

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