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

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Message-ID: <0de4a809-e965-d0ad-489f-5b011aa5bf89@redhat.com>
Date:   Wed, 29 Apr 2020 19:40:52 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     X86 ML <x86@...nel.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.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 29/04/20 19:28, Andy Lutomirski wrote:
> This seems functional, but I'm wondering if it could a bit simpler and
> more efficient if the data structure was a normal descriptor ring with
> the same number slots as whatever the maximum number of waiting pages
> is.  Then there would never need to be any notification from the guest
> back to the host, since there would always be room for a notification.

No, it would be much more complicated code for a slow path which is
already order of magnitudes slower than a vmexit.  It would also use
much more memory.

> It might be even better if a single unified data structure was used
> for both notifications.

That's a very bad idea since one is synchronous and one is asynchronous.
 Part of the proposal we agreed upon was to keep "page not ready"
synchronous while making "page ready" an interrupt.  The data structure
for "page not ready" will be #VE.

Paolo