Re: [PATCH v4 3/7] KVM: arm/arm64: Don't cache the timer IRQ level

[Marc Zyngier <marc.zyngier@arm.com>]

On Fri, Sep 15 2017 at  3:19:50 pm BST, Christoffer Dall <christoffer.dall@linaro.org> wrote:
> From: Christoffer Dall <cdall@linaro.org>
>
> The timer was modeled after a strict idea of modelling an interrupt line
> level in software, meaning that only transitions in the level needed to
> be reported to the VGIC.  This works well for the timer, because the
> arch timer code is in complete control of the device and can track the
> transitions of the line.
>
> However, as we are about to support using the HW bit in the VGIC not
> just for the timer, but also for VFIO which cannot track transitions of
> the interrupt line, we have to decide on an interface for level
> triggered mapped interrupts to the GIC, which both the timer and VFIO
> can use.
>
> VFIO only sees an asserting transition of the physical interrupt line,
> and tells the VGIC when that happens.  That means that part of the
> interrupt flow is offloaded to the hardware.
>
> To use the same interface for VFIO devices and the timer, we therefore
> have to change the timer (we cannot change VFIO because it doesn't know
> the details of the device it is assigning to a VM).
>
> Luckily, changing the timer is simple, we just need to stop 'caching'
> the line level, but instead let the VGIC know the state of the timer on
> every entry to the guest, and the VGIC can ignore notifications using
> its validate mechanism.
>
> Signed-off-by: Christoffer Dall <cdall@linaro.org>
> ---
>  virt/kvm/arm/arch_timer.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> index 8e89d63..2a5f877 100644
> --- a/virt/kvm/arm/arch_timer.c
> +++ b/virt/kvm/arm/arch_timer.c
> @@ -219,9 +219,10 @@ static void kvm_timer_update_irq(struct kvm_vcpu *vcpu, bool new_level,
>  	int ret;
>  
>  	timer_ctx->active_cleared_last = false;
> +	if (timer_ctx->irq.level != new_level)
> +		trace_kvm_timer_update_irq(vcpu->vcpu_id, timer_ctx->irq.irq,
> +					   new_level);
>  	timer_ctx->irq.level = new_level;
> -	trace_kvm_timer_update_irq(vcpu->vcpu_id, timer_ctx->irq.irq,
> -				   timer_ctx->irq.level);
>  
>  	if (likely(irqchip_in_kernel(vcpu->kvm))) {
>  		ret = kvm_vgic_inject_irq(vcpu->kvm, vcpu->vcpu_id,
> @@ -241,6 +242,7 @@ static void kvm_timer_update_state(struct kvm_vcpu *vcpu)
>  	struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
>  	struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
>  	struct arch_timer_context *ptimer = vcpu_ptimer(vcpu);
> +	bool level;
>  
>  	/*
>  	 * If userspace modified the timer registers via SET_ONE_REG before
> @@ -251,11 +253,11 @@ static void kvm_timer_update_state(struct kvm_vcpu *vcpu)
>  	if (unlikely(!timer->enabled))
>  		return;
>  
> -	if (kvm_timer_should_fire(vtimer) != vtimer->irq.level)
> -		kvm_timer_update_irq(vcpu, !vtimer->irq.level, vtimer);
> +	level = kvm_timer_should_fire(vtimer);
> +	kvm_timer_update_irq(vcpu, level, vtimer);
>  
> -	if (kvm_timer_should_fire(ptimer) != ptimer->irq.level)
> -		kvm_timer_update_irq(vcpu, !ptimer->irq.level, ptimer);
> +	level = kvm_timer_should_fire(ptimer);
> +	kvm_timer_update_irq(vcpu, level, ptimer);

Well, at this stage, you might as well fold the kvm_timer_should_fire()
into kvm_timer_update_irq() and from the level parameter. But I suspect
this is going to clash badly with your timer series?

>  }
>  
>  /* Schedule the background timer for the emulated timer. */

Otherwise:

Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>

	M.
-- 
Jazz is not dead, it just smell funny.