Subject: [PATCH 1/3] arm/arm64: KVM: Fix arch timer behavior for disabled interrupts

From: [email-masked] (Eric Auger)
Date: Mon, 19 Oct 2015 15:07:16 +0200

Hi Christoffer,
On 10/17/2015 10:30 PM, Christoffer Dall wrote:
> We have an interesting issue when the guest disables the timer interrupt
> on the VGIC, which happens when turning VCPUs off using PSCI, for
> example.
> 
> The problem is that because the guest disables the virtual interrupt at
> the VGIC level, we never inject interrupts to the guest and therefore
> never mark the interrupt as active on the physical distributor.  The
> host also never takes the timer interrupt (we only use the timer device
> to trigger a guest exit and everything else is done in software), so the
> interrupt does not become active through normal means.
> 
> The result is that we keep entering the guest with a programmed timer
> that will always fire as soon as we context switch the hardware timer
> state and run the guest, preventing forward progress for the VCPU.
> 
> Since the active state on the physical distributor is really part of the
> timer logic, it is the job of our virtual arch timer driver to manage
> this state.
> 
> The timer->map->active boolean field indicates whether we have signalled
> this interrupt to the vgic and if that interrupt is still pending or
> active.  As long as that is the case, the hardware doesn't have to
> generate physical interrupts and therefore we mark the interrupt as
> active on the physical distributor.
> 
> Cc: Marc Zyngier <[email-masked]>
> Reported-by: Lorenzo Pieralisi <[email-masked]>
> Signed-off-by: Christoffer Dall <[email-masked]>
> ---
>  virt/kvm/arm/arch_timer.c | 19 +++++++++++++++++++
>  virt/kvm/arm/vgic.c       | 43 +++++++++++--------------------------------
>  2 files changed, 30 insertions(+), 32 deletions(-)
> 
> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> index 48c6e1a..b9d3a32 100644
> --- a/virt/kvm/arm/arch_timer.c
> +++ b/virt/kvm/arm/arch_timer.c
> @@ -137,6 +137,8 @@ bool kvm_timer_should_fire(struct kvm_vcpu *vcpu)
>  void kvm_timer_flush_hwstate(struct kvm_vcpu *vcpu)
>  {
>  	struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
> +	bool phys_active;
> +	int ret;
>  
>  	/*
>  	 * We're about to run this vcpu again, so there is no need to
> @@ -151,6 +153,23 @@ void kvm_timer_flush_hwstate(struct kvm_vcpu *vcpu)
>  	 */
>  	if (kvm_timer_should_fire(vcpu))
>  		kvm_timer_inject_irq(vcpu);
> +
> +	/*
> +	 * We keep track of whether the edge-triggered interrupt has been
> +	 * signalled to the vgic/guest, and if so, we mask the interrupt and
> +	 * the physical distributor to prevent the timer from raising a
> +	 * physical interrupt whenever we run a guest, preventing forward
> +	 * VCPU progress.
In practice don't you simply mark the IRQ as active at GIC physical
distributor level, hence preventing the same IRQ from hitting again
> +	 */
> +	if (kvm_vgic_get_phys_irq_active(timer->map))
> +		phys_active = true;
> +	else
> +		phys_active = false;
> +
> +	ret = irq_set_irqchip_state(timer->map->irq,
> +				    IRQCHIP_STATE_ACTIVE,
> +				    phys_active);

physical distributor state is set in arch timer flush. It relates to a
shared device behavior so I find it natural to do it there.

However the map->active is set in arch_timer IRQ injection and unset in
vgic sync. Why not doing the set in kvm_vgic_inject_mapped_irq?

> +	WARN_ON(ret);
>  }
>  
>  /**
> diff --git a/virt/kvm/arm/vgic.c b/virt/kvm/arm/vgic.c
> index 596455a..ea21bc2 100644
> --- a/virt/kvm/arm/vgic.c
> +++ b/virt/kvm/arm/vgic.c
> @@ -1092,6 +1092,15 @@ static void vgic_retire_lr(int lr_nr, int irq, struct kvm_vcpu *vcpu)
>  	struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
>  	struct vgic_lr vlr = vgic_get_lr(vcpu, lr_nr);
>  
> +	/*
> +	 * We must transfer the pending state back to the distributor before
> +	 * retiring the LR, otherwise we may loose edge-triggered interrupts.
> +	 */
> +	if (vlr.state & LR_STATE_PENDING) {
> +		vgic_dist_irq_set_pending(vcpu, irq);
> +		vlr.hwirq = 0;
> +	}
That fix applies to any edge-sensitive IRQ, ie. not especially the
timer's one? In the positive shouldn't you precise this in the commit
msg too?

Best Regards

Eric

> +
>  	vlr.state = 0;
>  	vgic_set_lr(vcpu, lr_nr, vlr);
>  	clear_bit(lr_nr, vgic_cpu->lr_used);
> @@ -1241,7 +1250,7 @@ static void __kvm_vgic_flush_hwstate(struct kvm_vcpu *vcpu)
>  	struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
>  	struct vgic_dist *dist = &vcpu->kvm->arch.vgic;
>  	unsigned long *pa_percpu, *pa_shared;
> -	int i, vcpu_id, lr, ret;
> +	int i, vcpu_id;
>  	int overflow = 0;
>  	int nr_shared = vgic_nr_shared_irqs(dist);
>  
> @@ -1296,31 +1305,6 @@ epilog:
>  		 */
>  		clear_bit(vcpu_id, dist->irq_pending_on_cpu);
>  	}
> -
> -	for (lr = 0; lr < vgic->nr_lr; lr++) {
> -		struct vgic_lr vlr;
> -
> -		if (!test_bit(lr, vgic_cpu->lr_used))
> -			continue;
> -
> -		vlr = vgic_get_lr(vcpu, lr);
> -
> -		/*
> -		 * If we have a mapping, and the virtual interrupt is
> -		 * presented to the guest (as pending or active), then we must
> -		 * set the state to active in the physical world. See
> -		 * Documentation/virtual/kvm/arm/vgic-mapped-irqs.txt.
> -		 */
> -		if (vlr.state & LR_HW) {
> -			struct irq_phys_map *map;
> -			map = vgic_irq_map_search(vcpu, vlr.irq);
> -
> -			ret = irq_set_irqchip_state(map->irq,
> -						    IRQCHIP_STATE_ACTIVE,
> -						    true);
> -			WARN_ON(ret);
> -		}
> -	}
>  }
>  
>  static bool vgic_process_maintenance(struct kvm_vcpu *vcpu)
> @@ -1430,13 +1414,8 @@ static int vgic_sync_hwirq(struct kvm_vcpu *vcpu, struct vgic_lr vlr)
>  
>  	WARN_ON(ret);
>  
> -	if (map->active) {
> -		ret = irq_set_irqchip_state(map->irq,
> -					    IRQCHIP_STATE_ACTIVE,
> -					    false);
> -		WARN_ON(ret);
> +	if (map->active)
>  		return 0;
> -	}
>  
>  	return 1;
>  }
>