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 11:27:04 +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 10:40, Vitaly Kuznetsov wrote:
>>  I think in that case
>> kvm_check_async_pf_completion will refuse to make progress.
>> You need to make this bit stateful (e.g. 1 = async PF in progress, 0 =
>> not in progress), and check that for page ready notifications instead of
>> EFLAGS.IF.  
>> This probably means that;
>>
>> - it might be simpler to move it to the vector MSR
> I didn't want to merge 'ACK' with the vector MSR as it forces the guest
> to remember the setting. It doesn't matter at all for Linux as we
> hardcode the interrupt number but I can imaging an OS assigning IRQ
> numbers dynamically, it'll need to keep record to avoid doing rdmsr.

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.

>> - it's definitely much simpler to remove the #PF-based mechanism for
>> injecting page ready notifications.
> Yea, the logic in kvm_can_do_async_pf()/kvm_can_deliver_async_pf()
> becomes cumbersome. If we are to drop #PF-based mechanism I'd split it
> completely from the remaining synchronious #PF for page-not-present:
> basically, we only need to check that the slot (which we agreed becomes
> completely separate) is empty, interrupt/pending expception/... state
> becomes irrelevant.

Yes, that's a good point.

Paolo