Subject: Re: [PATCH 2/3] kvm: arm: VGIC: Scan all IRQs when interrupt group gets enabled

From: Marc Zyngier <[email-masked]>
Date: Sun, 10 Nov 2019 14:29:14 +0000

On Fri,  8 Nov 2019 17:49:51 +0000
Andre Przywara <[email-masked]> wrote:

> Our current VGIC emulation code treats the "EnableGrpX" bits in GICD_CTLR
> as a single global interrupt delivery switch, where in fact the GIC
> architecture asks for this being separate for the two interrupt groups.
> 
> To implement this properly, we have to slightly adjust our design, to
> *not* let IRQs from a disabled interrupt group be added to the ap_list.
> 
> As a consequence, enabling one group requires us to re-evaluate every
> pending IRQ and potentially add it to its respective ap_list. Similarly
> disabling an interrupt group requires pending IRQs to be removed from
> the ap_list (as long as they have not been activated yet).
> 
> Implement a rather simple, yet not terribly efficient algorithm to
> achieve this: For each VCPU we iterate over all IRQs, checking for
> pending ones and adding them to the list. We hold the ap_list_lock
> for this, to make this atomic from a VCPU's point of view.
> 
> When an interrupt group gets disabled, we can't directly remove affected
> IRQs from the ap_list, as a running VCPU might have already activated
> them, which wouldn't be immediately visible to the host.
> Instead simply kick all VCPUs, so that they clean their ap_list's
> automatically when running vgic_prune_ap_list().
> 
> Signed-off-by: Andre Przywara <[email-masked]>
> ---
>  virt/kvm/arm/vgic/vgic.c | 88 ++++++++++++++++++++++++++++++++++++----
>  1 file changed, 80 insertions(+), 8 deletions(-)
> 
> diff --git a/virt/kvm/arm/vgic/vgic.c b/virt/kvm/arm/vgic/vgic.c
> index 3b88e14d239f..28d9ff282017 100644
> --- a/virt/kvm/arm/vgic/vgic.c
> +++ b/virt/kvm/arm/vgic/vgic.c
> @@ -339,6 +339,38 @@ int vgic_dist_enable_group(struct kvm *kvm, int group, bool status)
>  	return 0;
>  }
>  
> +/*
> + * Check whether a given IRQs need to be queued to this ap_list, and do
> + * so if that's the case.
> + * Requires the ap_list_lock to be held (but not the irq lock).
> + *
> + * Returns 1 if that IRQ has been added to the ap_list, and 0 if not.
> + */
> +static int queue_enabled_irq(struct kvm *kvm, struct kvm_vcpu *vcpu,
> +			     int intid)

true/false seems better than 1/0.

> +{
> +	struct vgic_irq *irq = vgic_get_irq(kvm, vcpu, intid);
> +	int ret = 0;
> +
> +	raw_spin_lock(&irq->irq_lock);
> +	if (!irq->vcpu && vcpu == vgic_target_oracle(irq)) {
> +		/*
> +		 * Grab a reference to the irq to reflect the
> +		 * fact that it is now in the ap_list.
> +		 */
> +		vgic_get_irq_kref(irq);
> +		list_add_tail(&irq->ap_list,
> +			      &vcpu->arch.vgic_cpu.ap_list_head);

Two things:
- This should be the job of vgic_queue_irq_unlock. Why are you
  open-coding it?
- What if the interrupt isn't pending? Non-pending, non-active
  interrupts should not be on the AP list!

> +		irq->vcpu = vcpu;
> +
> +		ret = 1;
> +	}
> +	raw_spin_unlock(&irq->irq_lock);
> +	vgic_put_irq(kvm, irq);
> +
> +	return ret;
> +}
> +
>  /*
>   * The group enable status of at least one of the groups has changed.
>   * If enabled is true, at least one of the groups got enabled.
> @@ -346,17 +378,57 @@ int vgic_dist_enable_group(struct kvm *kvm, int group, bool status)
>   */
>  void vgic_rescan_pending_irqs(struct kvm *kvm, bool enabled)
>  {
> +	int cpuid;
> +	struct kvm_vcpu *vcpu;
> +
>  	/*
> -	 * TODO: actually scan *all* IRQs of the VM for pending IRQs.
> -	 * If a pending IRQ's group is now enabled, add it to its ap_list.
> -	 * If a pending IRQ's group is now disabled, kick the VCPU to
> -	 * let it remove this IRQ from its ap_list. We have to let the
> -	 * VCPU do it itself, because we can't know the exact state of an
> -	 * IRQ pending on a running VCPU.
> +	 * If no group got enabled, we only have to potentially remove
> +	 * interrupts from ap_lists. We can't do this here, because a running
> +	 * VCPU might have ACKed an IRQ already, which wouldn't immediately
> +	 * be reflected in the ap_list.
> +	 * So kick all VCPUs, which will let them re-evaluate their ap_lists
> +	 * by running vgic_prune_ap_list(), removing no longer enabled
> +	 * IRQs.
> +	 */
> +	if (!enabled) {
> +		vgic_kick_vcpus(kvm);
> +
> +		return;
> +	}
> +
> +	/*
> +	 * At least one group went from disabled to enabled. Now we need
> +	 * to scan *all* IRQs of the VM for newly group-enabled IRQs.
> +	 * If a pending IRQ's group is now enabled, add it to the ap_list.
> +	 *
> +	 * For each VCPU this needs to be atomic, as we need *all* newly
> +	 * enabled IRQs in be in the ap_list to determine the highest
> +	 * priority one.
> +	 * So grab the ap_list_lock, then iterate over all private IRQs and
> +	 * all SPIs. Once the ap_list is updated, kick that VCPU to
> +	 * forward any new IRQs to the guest.
>  	 */
> +	kvm_for_each_vcpu(cpuid, vcpu, kvm) {
> +		unsigned long flags;
> +		int i;
>  
> -	 /* For now just kick all VCPUs, as the old code did. */
> -	vgic_kick_vcpus(kvm);
> +		raw_spin_lock_irqsave(&vcpu->arch.vgic_cpu.ap_list_lock, flags);
> +
> +		for (i = 0; i < VGIC_NR_PRIVATE_IRQS; i++)
> +			queue_enabled_irq(kvm, vcpu, i);
> +
> +		for (i = VGIC_NR_PRIVATE_IRQS;
> +		     i < kvm->arch.vgic.nr_spis + VGIC_NR_PRIVATE_IRQS; i++)
> +			queue_enabled_irq(kvm, vcpu, i);

On top of my questions above, what happens to LPIs? And if a group has
been disabled, how do you retire these interrupts from the AP list?

> +
> +                raw_spin_unlock_irqrestore(&vcpu->arch.vgic_cpu.ap_list_lock,
> +                                           flags);
> +
> +		if (kvm_vgic_vcpu_pending_irq(vcpu)) {
> +			kvm_make_request(KVM_REQ_IRQ_PENDING, vcpu);
> +			kvm_vcpu_kick(vcpu);
> +		}
> +	}
>  }
>  
>  bool vgic_dist_group_enabled(struct kvm *kvm, int group)

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...

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