Re: [PATCH v3 09/20] KVM: arm/arm64: Use separate timer for phys timer emulation

Christoffer Dall
2017-10-19T07:38:09Z

On Mon, Oct 09, 2017 at 06:23:45PM +0100, Marc Zyngier wrote:
> On 23/09/17 01:41, Christoffer Dall wrote:
> > We were using the same hrtimer for emulating the physical timer and for
> > making sure a blocking VCPU thread would be eventually woken up. That
> > worked fine in the previous arch timer design, but as we are about to
> > actually use the soft timer expire function for the physical timer
> > emulation, change the logic to use a dedicated hrtimer.
> >
> > This has the added benefit of not having to cancel any work in the sync
> > path, which in turn allows us to run the flush and sync with IRQs
> > disabled.
> >
> > Signed-off-by: Christoffer Dall <cdall@linaro.org>
> > ---
> > include/kvm/arm_arch_timer.h | 3 +++
> > virt/kvm/arm/arch_timer.c | 18 ++++++++++++++----
> > 2 files changed, 17 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/kvm/arm_arch_timer.h b/include/kvm/arm_arch_timer.h
> > index dcbb2e1..16887c0 100644
> > --- a/include/kvm/arm_arch_timer.h
> > +++ b/include/kvm/arm_arch_timer.h
> > @@ -47,6 +47,9 @@ struct arch_timer_cpu {
> > /* Work queued with the above timer expires */
> > struct work_struct expired;
> >
> > + /* Physical timer emulation */
> > + struct hrtimer phys_timer;
> > +
> > /* Background timer active */
> > bool armed;
> >
> > diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> > index c2e8326..7f87099 100644
> > --- a/virt/kvm/arm/arch_timer.c
> > +++ b/virt/kvm/arm/arch_timer.c
> > @@ -178,6 +178,12 @@ static enum hrtimer_restart kvm_bg_timer_expire(struct hrtimer *hrt)
> > return HRTIMER_NORESTART;
> > }
> >
> > +static enum hrtimer_restart kvm_phys_timer_expire(struct hrtimer *hrt)
> > +{
> > + WARN(1, "Timer only used to ensure guest exit - unexpected event.");
> > + return HRTIMER_NORESTART;
> > +}
> > +
>
> So what prevents this handler from actually firing? Is it that we cancel
> the hrtimer while interrupts are still disabled, hence the timer never
> fires? If that's the intention, then this patch is slightly out of
> place, as we haven't moved the timer sync within the irq_disable() section.
>
> Or am I missing something obvious?
>

No you're not missing anything, indeed, that is broken. I think I had
in the back of my mind that we disable stuff in the world-switch still,
but that obviously doesn't apply to the soft timers.

I'll just move this patch following the next one where interrupts are
disabled.

Nice catch!

> > bool kvm_timer_should_fire(struct arch_timer_context *timer_ctx)
> > {
> > u64 cval, now;
> > @@ -255,7 +261,7 @@ static void kvm_timer_update_state(struct kvm_vcpu *vcpu)
> > }
> >
> > /* Schedule the background timer for the emulated timer. */
> > -static void kvm_timer_emulate(struct kvm_vcpu *vcpu,
> > +static void phys_timer_emulate(struct kvm_vcpu *vcpu,
> > struct arch_timer_context *timer_ctx)
> > {
> > struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
> > @@ -267,7 +273,7 @@ static void kvm_timer_emulate(struct kvm_vcpu *vcpu,
> > return;
> >
> > /* The timer has not yet expired, schedule a background timer */
> > - soft_timer_start(&timer->bg_timer, kvm_timer_compute_delta(timer_ctx));
> > + soft_timer_start(&timer->phys_timer, kvm_timer_compute_delta(timer_ctx));
> > }
> >
> > /*
> > @@ -424,7 +430,7 @@ void kvm_timer_flush_hwstate(struct kvm_vcpu *vcpu)
> > kvm_timer_update_state(vcpu);
> >
> > /* Set the background timer for the physical timer emulation. */
> > - kvm_timer_emulate(vcpu, vcpu_ptimer(vcpu));
> > + phys_timer_emulate(vcpu, vcpu_ptimer(vcpu));
> >
> > if (unlikely(!irqchip_in_kernel(vcpu->kvm)))
> > kvm_timer_flush_hwstate_user(vcpu);
> > @@ -447,7 +453,7 @@ void kvm_timer_sync_hwstate(struct kvm_vcpu *vcpu)
> > * This is to cancel the background timer for the physical timer
> > * emulation if it is set.
> > */
> > - soft_timer_cancel(&timer->bg_timer, &timer->expired);
> > + soft_timer_cancel(&timer->phys_timer, NULL);
>
> Right, that now explains the "work" test in one of the previous patches.
>

Yes, I've moved the addition of the test to this patch which actually
uses is.

> >
> > /*
> > * The guest could have modified the timer registers or the timer
> > @@ -507,6 +513,9 @@ void kvm_timer_vcpu_init(struct kvm_vcpu *vcpu)
> > hrtimer_init(&timer->bg_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
> > timer->bg_timer.function = kvm_bg_timer_expire;
> >
> > + hrtimer_init(&timer->phys_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
> > + timer->phys_timer.function = kvm_phys_timer_expire;
> > +
> > vtimer->irq.irq = default_vtimer_irq.irq;
> > ptimer->irq.irq = default_ptimer_irq.irq;
> > }
> > @@ -615,6 +624,7 @@ void kvm_timer_vcpu_terminate(struct kvm_vcpu *vcpu)
> > struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
> >
> > soft_timer_cancel(&timer->bg_timer, &timer->expired);
> > + soft_timer_cancel(&timer->phys_timer, NULL);
> > kvm_vgic_unmap_phys_irq(vcpu, vtimer->irq.irq);
> > }
> >
> >
>

Thanks,
-Christoffer