Re: [PATCH v3 18/20] KVM: arm/arm64: Avoid phys timer emulation in vcpu entry/exit

Christoffer Dall
2017-10-19T08:44:43Z

On Tue, Oct 10, 2017 at 10:45:15AM +0100, Marc Zyngier wrote:
> On Sat, Sep 23 2017 at 2:42:05 am BST, Christoffer Dall <cdall@linaro.org> wrote:
> > There is no need to schedule and cancel a hrtimer when entering and
> > exiting the guest, because we know when the physical timer is going to
> > fire when the guest programs it, and we can simply program the hrtimer
> > at that point.
> >
> > Now when the register modifications from the guest go through the
> > kvm_arm_timer_set/get_reg functions, which always call
> > kvm_timer_update_state(), we can simply consider the timer state in this
> > function and schedule and cancel the timers as needed.
> >
> > This avoids looking at the physical timer emulation state when entering
> > and exiting the VCPU, allowing for faster servicing of the VM when
> > needed.
> >
> > Signed-off-by: Christoffer Dall <cdall@linaro.org>
> > ---
> > virt/kvm/arm/arch_timer.c | 75 ++++++++++++++++++++++++++++++++---------------
> > 1 file changed, 51 insertions(+), 24 deletions(-)
> >
> > diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> > index 1f82c21..aa18a5d 100644
> > --- a/virt/kvm/arm/arch_timer.c
> > +++ b/virt/kvm/arm/arch_timer.c
> > @@ -199,7 +199,27 @@ static enum hrtimer_restart kvm_bg_timer_expire(struct hrtimer *hrt)
> >
> > static enum hrtimer_restart kvm_phys_timer_expire(struct hrtimer *hrt)
> > {
> > - WARN(1, "Timer only used to ensure guest exit - unexpected event.");
> > + struct arch_timer_context *ptimer;
> > + struct arch_timer_cpu *timer;
> > + struct kvm_vcpu *vcpu;
> > + u64 ns;
> > +
> > + timer = container_of(hrt, struct arch_timer_cpu, phys_timer);
> > + vcpu = container_of(timer, struct kvm_vcpu, arch.timer_cpu);
> > + ptimer = vcpu_ptimer(vcpu);
> > +
> > + /*
> > + * Check that the timer has really expired from the guest's
> > + * PoV (NTP on the host may have forced it to expire
> > + * early). If not ready, schedule for a later time.
> > + */
> > + ns = kvm_timer_compute_delta(ptimer);
> > + if (unlikely(ns)) {
> > + hrtimer_forward_now(hrt, ns_to_ktime(ns));
> > + return HRTIMER_RESTART;
> > + }
>
> Don't we already have a similar logic for the background timer (I must
> admit I've lost track of how we changed things in this series)? If so,
> can we make this common code?
>

I looked at it, but the functions are slightly different. In the
phys_timer, we just have to calculate the delta and figure out if we
need to restart it. In the bg_timer, we have to compute the delta of
both the vtimer and the ptimer, figure out the earliest one, and figure
out if we have to restart it. Therefore, kvm_timer_earliest_exp()
alrady calls kvm_timer_compute_delate() to share code, and the only code
we can really share is:

if (unlikely(ns)) {
hrtimer_forward_now(hrt, ns_to_ktime(ns));
return HRTIMER_RESTART;
}

The following code also cannot really be shared because in one case we
have to schedule work (the bg timer) and in the other case we have to
inject an irq (the phys timer). Ther alternative would be:

static enum hrtimer_restart kvm_soft_timer_expire(struct hrtimer *hrt, bool is_bg_timer)
{
struct arch_timer_context *ptimer;
struct arch_timer_cpu *timer;
struct kvm_vcpu *vcpu;
u64 ns;

if (is_bg_timer)
timer = container_of(hrt, struct arch_timer_cpu, bg_timer);
else
timer = container_of(hrt, struct arch_timer_cpu, phys_timer);
vcpu = container_of(timer, struct kvm_vcpu, arch.timer_cpu);
ptimer = vcpu_ptimer(vcpu);

/*
* Check that the timer has really expired from the guest's
* PoV (NTP on the host may have forced it to expire
* early). If we should have slept longer, restart it.
*/
if (is_bg_timer)
ns = kvm_timer_earliest_exp(vcpu);
else
ns = kvm_timer_compute_delta(ptimer);
if (unlikely(ns)) {
hrtimer_forward_now(hrt, ns_to_ktime(ns));
return HRTIMER_RESTART;
}

if (is_bg_timer)
schedule_work(&timer->expired);
else
kvm_timer_update_irq(vcpu, true, ptimer);
return HRTIMER_NORESTART;
}

But I prefer just having them separate.

> > +
> > + kvm_timer_update_irq(vcpu, true, ptimer);
> > return HRTIMER_NORESTART;
> > }
> >
> > @@ -253,24 +273,28 @@ static void kvm_timer_update_irq(struct kvm_vcpu *vcpu, bool new_level,
> > }
> >
> > /* Schedule the background timer for the emulated timer. */
> > -static void phys_timer_emulate(struct kvm_vcpu *vcpu,
> > - struct arch_timer_context *timer_ctx)
> > +static void phys_timer_emulate(struct kvm_vcpu *vcpu)
> > {
> > struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
> > + struct arch_timer_context *ptimer = vcpu_ptimer(vcpu);
> >
> > - if (kvm_timer_should_fire(timer_ctx))
> > - return;
> > -
> > - if (!kvm_timer_irq_can_fire(timer_ctx))
> > + /*
> > + * If the timer can fire now we have just raised the IRQ line and we
> > + * don't need to have a soft timer scheduled for the future. If the
> > + * timer cannot fire at all, then we also don't need a soft timer.
> > + */
> > + if (kvm_timer_should_fire(ptimer) || !kvm_timer_irq_can_fire(ptimer)) {
> > + soft_timer_cancel(&timer->phys_timer, NULL);
> > return;
> > + }
> >
> > - /* The timer has not yet expired, schedule a background timer */
> > - soft_timer_start(&timer->phys_timer, kvm_timer_compute_delta(timer_ctx));
> > + soft_timer_start(&timer->phys_timer, kvm_timer_compute_delta(ptimer));
> > }
> >
> > /*
> > - * Check if there was a change in the timer state (should we raise or lower
> > - * the line level to the GIC).
> > + * Check if there was a change in the timer state, so that we should either
> > + * raise or lower the line level to the GIC or schedule a background timer to
> > + * emulate the physical timer.
> > */
> > static void kvm_timer_update_state(struct kvm_vcpu *vcpu)
> > {
> > @@ -292,6 +316,8 @@ static void kvm_timer_update_state(struct kvm_vcpu *vcpu)
> >
> > if (kvm_timer_should_fire(ptimer) != ptimer->irq.level)
> > kvm_timer_update_irq(vcpu, !ptimer->irq.level, ptimer);
> > +
> > + phys_timer_emulate(vcpu);
> > }
> >
> > static void vtimer_save_state(struct kvm_vcpu *vcpu)
> > @@ -445,6 +471,9 @@ void kvm_timer_vcpu_load(struct kvm_vcpu *vcpu)
> >
> > if (has_vhe())
> > disable_el1_phys_timer_access();
> > +
> > + /* Set the background timer for the physical timer emulation. */
> > + phys_timer_emulate(vcpu);
> > }
> >
> > bool kvm_timer_should_notify_user(struct kvm_vcpu *vcpu)
> > @@ -480,12 +509,6 @@ void kvm_timer_flush_hwstate(struct kvm_vcpu *vcpu)
> >
> > if (unlikely(!timer->enabled))
> > return;
> > -
> > - if (kvm_timer_should_fire(ptimer) != ptimer->irq.level)
> > - kvm_timer_update_irq(vcpu, !ptimer->irq.level, ptimer);
> > -
> > - /* Set the background timer for the physical timer emulation. */
> > - phys_timer_emulate(vcpu, vcpu_ptimer(vcpu));
> > }
> >
> > void kvm_timer_vcpu_put(struct kvm_vcpu *vcpu)
> > @@ -500,6 +523,17 @@ void kvm_timer_vcpu_put(struct kvm_vcpu *vcpu)
> >
> > vtimer_save_state(vcpu);
> >
> > + /*
> > + * Cancel the physical timer emulation, because the only case where we
> > + * need it after a vcpu_put is in the context of a sleeping VCPU, and
> > + * in that case we already factor in the deadline for the physical
> > + * timer when scheduling the bg_timer.
> > + *
> > + * In any case, we re-schedule the hrtimer for the physical timer when
> > + * coming back to the VCPU thread in kvm_timer_vcpu_load().
> > + */
> > + soft_timer_cancel(&timer->phys_timer, NULL);
> > +
> > set_cntvoff(0);
> > }
> >
> > @@ -536,16 +570,9 @@ static void unmask_vtimer_irq(struct kvm_vcpu *vcpu)
> > */
> > void kvm_timer_sync_hwstate(struct kvm_vcpu *vcpu)
> > {
> > - struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
> > struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
> >
> > /*
> > - * This is to cancel the background timer for the physical timer
> > - * emulation if it is set.
> > - */
> > - soft_timer_cancel(&timer->phys_timer, NULL);
> > -
> > - /*
> > * If we entered the guest with the vtimer output asserted we have to
> > * check if the guest has modified the timer so that we should lower
> > * the line at this point.
>
> Otherwise:
>
> Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
>
Thanks,
-Christoffer