linux-kernel - Re: [PATCH RFC 3/6] KVM: x86: interrupt based APF page-ready event delivery

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Message-ID: <877dxxf5hu.fsf@vitty.brq.redhat.com>
Date:   Thu, 30 Apr 2020 15:49:33 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        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 3/6] KVM: x86: interrupt based APF page-ready event delivery

Peter Xu <peterx@...hat.com> writes:

> On Thu, Apr 30, 2020 at 10:31:32AM +0200, Vitaly Kuznetsov wrote:
>> as we need to write to two MSRs to configure the new mechanism ordering
>> becomes important. If the guest writes to ASYNC_PF_EN first to establish
>> the shared memory stucture the interrupt in ASYNC_PF2 is not yet set
>> (and AFAIR '0' is a valid interrupt!) so if an async pf happens
>> immediately after that we'll be forced to inject INT0 in the guest and
>> it'll get confused and linkely miss the event.
>> 
>> We can probably mandate the reverse sequence: guest has to set up
>> interrupt in ASYNC_PF2 first and then write to ASYNC_PF_EN (with both
>> bit 0 and bit 3). In that case the additional 'enable' bit in ASYNC_PF2
>> seems redundant. This protocol doesn't look too complex for guests to
>> follow.
>
> Yep looks good.  We should also update the document too about the fact.
>

Of course. It will also mention the fact that #PF-based mechanism is
now depreceted and unless guest opts into interrupt based delivery
async_pf is not functional (except for APF_HALT).

-- 
Vitaly