linux-kernel - Re: [PATCH 4/8] KVM: x86: interrupt based APF page-ready event delivery

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Message-ID: <87zhabdjlm.fsf@vitty.brq.redhat.com>
Date:   Thu, 14 May 2020 10:08:37 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        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>,
        Gavin Shan <gshan@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/8] KVM: x86: interrupt based APF page-ready event delivery

Vivek Goyal <vgoyal@...hat.com> writes:

> On Wed, May 13, 2020 at 04:23:55PM +0200, Vitaly Kuznetsov wrote:
>
> [..]
>> >> Also,
>> >> kdump kernel may not even support APF so it will get very confused when
>> >> APF events get delivered.
>> >
>> > New kernel can just ignore these events if it does not support async
>> > pf? 
>> >
>> > This is somewhat similar to devices still doing interrupts in new
>> > kernel. And solution for that seemed to be doing a "reset" of devices
>> > in new kernel. We probably need similar logic where in new kernel
>> > we simply disable "async pf" so that we don't get new notifications.
>> 
>> Right and that's what we're doing - just disabling new notifications.
>
> Nice.
>
> So why there is a need to deliver "page ready" notifications
> to guest after guest has disabled async pf. Atleast kdump does not
> seem to need it. It will boot into second kernel anyway, irrespective
> of the fact whether it receives page ready or not.

We don't deliver anything to the guest after it disables APF (neither
'page ready' for what was previously missing, nor 'page not ready' for
new faults), kvm_arch_can_inject_async_page_present() is just another
misnomer, it should be named something like
'kvm_arch_can_unqueue_async_page_present()' meaning that 'page ready'
notification can be 'unqueued' from internal KVM queue. We will either
deliver it (when guest has APF enabled) or just drop it (when guest has
APF disabled). The only case when it has to stay in the queue is when
guest has APF enabled and the slot is still busy (so it didn't get to
process a previously delivered notification). We will try to deliver it
again after guest writes to MSR_KVM_ASYNC_PF_ACK.

-- 
Vitaly