linux-kernel - Re: [PATCH v2 05/10] KVM: x86: interrupt based APF 'page ready' event delivery

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Message-ID: <dcdda87c-cf2f-da6f-3166-e2d0bfefce06@redhat.com>
Date:   Tue, 9 Jun 2020 22:31:24 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vivek Goyal <vgoyal@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Jim Mattson <jmattson@...gle.com>,
        Gavin Shan <gshan@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 05/10] KVM: x86: interrupt based APF 'page ready' event
 delivery

On 09/06/20 21:10, Vivek Goyal wrote:
> Hi Vitaly,
> 
> Have a question about page ready events. 
> 
> Now we deliver PAGE_NOT_PRESENT page faults only if guest is not in
> kernel mode. So say kernel tried to access a page and we halted cpu.
> When page is available, we will inject page_ready interrupt. At
> that time we don't seem to check whether page_not_present was injected
> or not. 
> 
> IOW, we seem to deliver page_ready irrespective of the fact whether
> PAGE_NOT_PRESENT was delivered or not. And that means we will be
> sending page present tokens to guest. Guest will not have a state
> associated with that token and think that page_not_present has
> not been delivered yet and allocate an element in hash table for
> future page_not_present event. And that will lead to memory leak
> and token conflict etc.

Yes, and this is https://bugzilla.kernel.org/show_bug.cgi?id=208081
which I was looking at right today.

> While setting up async pf, should we keep track whether associated
> page_not_present was delivered to guest or not and deliver page_ready
> accordingly.

Yes, I think so.

Paolo