linux-kernel - Re: [PATCH RFC 4/6] KVM: x86: acknowledgment mechanism for async pf page ready notifications

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Date:   Thu, 30 Apr 2020 13:43:13 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, x86@...nel.org,
        kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH RFC 4/6] KVM: x86: acknowledgment mechanism for async pf
 page ready notifications

On 30/04/20 13:33, Vitaly Kuznetsov wrote:
>> I would expect that it needs to keep it in a global variable anyway, but
>> yes this is a good point.  You can also keep the ACK MSR and store the
>> pending bit in the other MSR, kind of like you have separate ISR and EOI
>> registers in the LAPIC.
>>
> Honestly I was inspired by Hyper-V's HV_X64_MSR_EOM MSR as the protocol
> we're trying to come up with here is very similar to HV messaging)

Oh, that's true actually.

> I'm not exactly sure why we need the pending bit after we drop #PF. When
> we call kvm_check_async_pf_completion() from MSR_KVM_ASYNC_PF_ACK write
> it will (in case there are page ready events in the queue) check if the
> slot is empty, put one there and raise IRQ regardless of guest's current
> state. It may or may not get injected immediately but we don't care.> The second invocation of kvm_check_async_pf_completion() from vcpu_run()
> will just go away.

You're right, you can just use the value in the guest to see if the
guest is ready.  This is also similar to how #VE handles re-entrancy,
however because this is an interrupt we have IF to delay the IRQ until
after the interrupt handler has finished.

By dropping the #PF page ready case, we can also drop the ugly case
where WRMSR injects a page ready page fault even if IF=0.  That one is
safe on Linux, but Andy didn't like it.

Paolo