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

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Date:   Fri, 22 Sep 2023 10:29:25 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     paul@....org, kvm <kvm@...r.kernel.org>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        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,
        "Griffoul, Fred" <fgriffo@...zon.com>
Subject: Re: [PATCH] KVM: x86: Use fast path for Xen timer delivery

On Fri, 2023-09-22 at 10:22 +0100, Paul Durrant wrote:
> On 22/09/2023 10:20, David Woodhouse wrote:
> > From: David Woodhouse <dwmw@...zon.co.uk>
> > 
> > Most of the time there's no need to kick the vCPU and deliver the timer
> > event through kvm_xen_inject_timer_irqs(). Use kvm_xen_set_evtchn_fast()
> > directly from the timer callback, and only fall back to the slow path
> > when it's necessary to do so.
> > 
> > This gives a significant improvement in timer latency testing (using
> > nanosleep() for various periods and then measuring the actual time
> > elapsed).
> > 
> > Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> > ---
> >   arch/x86/kvm/xen.c | 17 ++++++++++++++---
> >   1 file changed, 14 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> > index 40edf4d1974c..66c4cf93a55c 100644
> > --- a/arch/x86/kvm/xen.c
> > +++ b/arch/x86/kvm/xen.c
> > @@ -134,12 +134,23 @@ static enum hrtimer_restart xen_timer_callback(struct hrtimer *timer)
> >   {
> >         struct kvm_vcpu *vcpu = container_of(timer, struct kvm_vcpu,
> >                                              arch.xen.timer);
> > +       struct kvm_xen_evtchn e;
> > +       int rc;
> > +
> >         if (atomic_read(&vcpu->arch.xen.timer_pending))
> >                 return HRTIMER_NORESTART;
> >   
> > -       atomic_inc(&vcpu->arch.xen.timer_pending);
> > -       kvm_make_request(KVM_REQ_UNBLOCK, vcpu);
> > -       kvm_vcpu_kick(vcpu);
> > +       e.vcpu_id = vcpu->vcpu_id;
> > +       e.vcpu_idx = vcpu->vcpu_idx;
> > +       e.port = vcpu->arch.xen.timer_virq;
> 
> Don't you need to check the port for validity? The VMM may not have set 
> it (and hence be delivering timer VIRQ events itself).

Nah, the kvm_xen_timer_enabled() check which gates all the kernel
interception of timer hypercalls is already testing precisely that.

If the TIMER_VIRQ isn't set, none of this code runs. And when userspace
tears down TIMER_VIRQ, the hrtimer is cancelled. So none of this code
(then) runs.

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