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

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Message-ID: <20200430132805.GB40678@xz-x1>
Date:   Thu, 30 Apr 2020 09:28:05 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...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

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.

Thanks,

-- 
Peter Xu