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: <2b8d0e27-a893-b090-7ecd-df17eefdf6f1@redhat.com>
Date:   Tue, 25 May 2021 09:11:10 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...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

On 25/05/21 08:23, Vitaly Kuznetsov wrote:
>> 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).

Ok, if enable<->disable APICv becomes an issue we can also consider 
disabling APICv forever if AutoEOI is ever used.

Paolo