linux-kernel - Re: [PATCH v2 10/10] KVM: x86: deprecate KVM_ASYNC_PF_SEND

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Message-ID: <87zh9s2txj.fsf@vitty.brq.redhat.com>
Date:   Thu, 28 May 2020 13:14:00 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        x86@...nel.org
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Jim Mattson <jmattson@...gle.com>,
        Vivek Goyal <vgoyal@...hat.com>, Gavin Shan <gshan@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 10/10] KVM: x86: deprecate KVM_ASYNC_PF_SEND_ALWAYS

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

> On 25/05/20 16:41, Vitaly Kuznetsov wrote:
>> Concerns were expressed around APF events delivery when CPU is not
>> in user mode (KVM_ASYNC_PF_SEND_ALWAYS), e.g.
>> https://lore.kernel.org/kvm/ed71d0967113a35f670a9625a058b8e6e0b2f104.1583547991.git.luto@kernel.org/
>> 
>> 'Page ready' events are already free from '#PF abuse' but 'page not ready'
>> notifications still go through #PF (to be changed in the future). Make the
>> probability of running into issues when APF collides with regular #PF lower
>> by deprecating KVM_ASYNC_PF_SEND_ALWAYS. The feature doesn't seem to be
>> important enough for any particular workload to notice the difference.
>
> This has been disabled already in guest code, but I don't see a
> particular reason to deprecate it in the host too.  Supporting it on the
> host is just one line of code; if it's a problem *for the guest*, you
> just don't use KVM_ASYNC_PF_SEND_ALWAYS.
>
> Also, note that #VE will always be delivered to the guest even at CPL0;
> the decision on whether to do sync or async page fault at CPL0 will move
> to the guest, but enabling #VE will probably _require_ the
> KVM_ASYNC_PF_SEND_ALWAYS bit to be set (and KVM_ASYNC_PF_DELIVERY_AS_INT
> as well).

I actually missed the fact that guest side is already disabled (I can
see it now in the queue), feel free to ignore this patch then.

-- 
Vitaly