linux-kernel - Re: [PATCH v3] KVM: x86: Use fast path for Xen timer delivery

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Message-ID: <c57dc32ee6ef31854cac372fde8dc097aa2dab40.camel@infradead.org>
Date:   Mon, 02 Oct 2023 18:05:40 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm <kvm@...r.kernel.org>, Paul Durrant <paul@....org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] KVM: x86: Use fast path for Xen timer delivery

On Mon, 2023-10-02 at 10:00 -0700, Sean Christopherson wrote:
> 
> >         case KVM_XEN_VCPU_ATTR_TYPE_TIMER:
> > +               /*
> > +                * Ensure a consistent snapshot of state is captured, with a
> > +                * timer either being pending, or the event channel delivered
> > +                * to the corresponding bit in the shared_info. Not still
> > +                * lurking in the timer_pending flag for deferred delivery.
> > +                * Purely as an optimisation, if the timer_expires field is
> > +                * zero, that means the timer isn't active (or even in the
> > +                * timer_pending flag) and there is no need to cancel it.
> > +                */
> 
> Ah, kvm_xen_start_timer() zeros timer_pending.
> 
> Given that, shouldn't it be impossible for xen_timer_callback() to observe a
> non-zero timer_pending value?  E.g. couldn't this code WARN?
> 
>         if (atomic_read(&vcpu->arch.xen.timer_pending))
>                 return HRTIMER_NORESTART;
> 
> Obviously not a blocker for this patch, I'm mostly just curious to know if I'm
> missing something.

Yes, I believe that is true.

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