linux-kernel - Re: [PATCH v2 5/5] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use

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Message-ID: <87fsyb8h7s.fsf@vitty.brq.redhat.com>
Date:   Tue, 25 May 2021 08:23:19 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Kechen Lu <kechenl@...dia.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/5] KVM: x86: hyper-v: Deactivate APICv only when
 AutoEOI feature is in use

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 18/05/21 16:43, Vitaly Kuznetsov wrote:
>> APICV_INHIBIT_REASON_HYPERV is currently unconditionally forced upon
>> SynIC activation as SynIC's AutoEOI is incompatible with APICv/AVIC. It is,
>> however, possible to track whether the feature was actually used by the
>> guest and only inhibit APICv/AVIC when needed.
>> 
>> TLFS suggests a dedicated 'HV_DEPRECATING_AEOI_RECOMMENDED' flag to let
>> Windows know that AutoEOI feature should be avoided. While it's up to
>> KVM userspace to set the flag, KVM can help a bit by exposing global
>> APICv/AVIC enablement: in case APICv/AVIC usage is impossible, AutoEOI
>> is still preferred.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>
> Should it also disable APICv unconditionally if 
> HV_DEPRECATING_AEOI_RECOMMENDED is not in the guest CPUID?  That should 
> avoid ping-pong between enabled and disabled APICv even in pathological 
> cases that we cannot think about.

When you run Hyper-V on KVM it doesn't use SynIC (let alone AutoEOI) but
we still inhibit APICv unconditionally. The patch as-is improves this
without any userspace changes required and I see it as a benefit. Going
forward, we will definitely add something like 'hv-synic-noaeoi' to QEMU
to make non-nesting setups benefit too but it'll take a while for this
option to propagate to real world configurations (sigh).

-- 
Vitaly