Subject: Re: [PATCH v6 04/39] KVM: arm64: vgic: Split out mapping IRQs and setting irq_ops

From: Marc Zyngier <[email-masked]>
Date: Tue, 17 Mar 2026 16:00:56 +0000

On Tue, 17 Mar 2026 11:40:59 +0000,
Sascha Bischoff <[email-masked]> wrote:
> 
> Prior to this change, the act of mapping a virtual IRQ to a physical
> one also set the irq_ops. Unmapping then reset the irq_ops to NULL. So
> far, this has been fine and hasn't caused any major issues.
> 
> Now, however, as GICv5 support is being added to KVM, it has become
> apparent that conflating mapping/unmapping IRQs and setting/clearing
> irq_ops can cause issues. The reason is that the upcoming GICv5
> support introduces a set of default irq_ops for PPIs, and removing
> this when unmapping will cause things to break rather horribly.
> 
> Split out the mapping/unmapping of IRQs from the setting/clearing of
> irq_ops. The arch timer code is updated to set the irq_ops following a
> successful map. The irq_ops are intentionally not removed again on an
> unmap as the only irq_op introduced by the arch timer only takes
> effect if the hw bit in struct vgic_irq is set. Therefore, it is safe
> to leave this in place, and it avoids additional complexity when GICv5
> support is introduced.
> 
> Signed-off-by: Sascha Bischoff <[email-masked]>
> ---
>  arch/arm64/kvm/arch_timer.c | 32 ++++++++++++++++++-------------
>  arch/arm64/kvm/vgic/vgic.c  | 38 +++++++++++++++++++++++++++++++------
>  include/kvm/arm_vgic.h      |  5 ++++-
>  3 files changed, 55 insertions(+), 20 deletions(-)
> 
> diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c
> index 600f250753b45..1f536dd5978d4 100644
> --- a/arch/arm64/kvm/arch_timer.c
> +++ b/arch/arm64/kvm/arch_timer.c
> @@ -740,14 +740,17 @@ static void kvm_timer_vcpu_load_nested_switch(struct kvm_vcpu *vcpu,
>  
>  		ret = kvm_vgic_map_phys_irq(vcpu,
>  					    map->direct_vtimer->host_timer_irq,
> -					    timer_irq(map->direct_vtimer),
> -					    &arch_timer_irq_ops);
> -		WARN_ON_ONCE(ret);
> +					    timer_irq(map->direct_vtimer));
> +		if (!WARN_ON_ONCE(ret))
> +			kvm_vgic_set_irq_ops(vcpu, timer_irq(map->direct_vtimer),
> +					     &arch_timer_irq_ops);
> +
>  		ret = kvm_vgic_map_phys_irq(vcpu,
>  					    map->direct_ptimer->host_timer_irq,
> -					    timer_irq(map->direct_ptimer),
> -					    &arch_timer_irq_ops);
> -		WARN_ON_ONCE(ret);
> +					    timer_irq(map->direct_ptimer));
> +		if (!WARN_ON_ONCE(ret))
> +			kvm_vgic_set_irq_ops(vcpu, timer_irq(map->direct_ptimer),
> +					     &arch_timer_irq_ops);

Do we really need this eager setting of ops? Given that nothing seems
to clear them, why can't we just set the ops at vcpu init time? Given
that this is a pretty hot path (on each exception/exception return
between L2 and L1), the least we do here, the better.

>  	}
>  }
>  
> @@ -1565,20 +1568,23 @@ int kvm_timer_enable(struct kvm_vcpu *vcpu)
>  
>  	ret = kvm_vgic_map_phys_irq(vcpu,
>  				    map.direct_vtimer->host_timer_irq,
> -				    timer_irq(map.direct_vtimer),
> -				    &arch_timer_irq_ops);
> +				    timer_irq(map.direct_vtimer));
>  	if (ret)
>  		return ret;
>  
> +	kvm_vgic_set_irq_ops(vcpu, timer_irq(map.direct_vtimer),
> +			     &arch_timer_irq_ops);
> +
>  	if (map.direct_ptimer) {
>  		ret = kvm_vgic_map_phys_irq(vcpu,
>  					    map.direct_ptimer->host_timer_irq,
> -					    timer_irq(map.direct_ptimer),
> -					    &arch_timer_irq_ops);
> -	}
> +					    timer_irq(map.direct_ptimer));
> +		if (ret)
> +			return ret;
>  
> -	if (ret)
> -		return ret;
> +		kvm_vgic_set_irq_ops(vcpu, timer_irq(map.direct_ptimer),
> +				     &arch_timer_irq_ops);
> +	}

which would mean moving this to kvm_timer_vcpu_init().

>  
>  no_vgic:
>  	timer->enabled = 1;
> diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
> index e22b79cfff965..e37c640d74bcf 100644
> --- a/arch/arm64/kvm/vgic/vgic.c
> +++ b/arch/arm64/kvm/vgic/vgic.c
> @@ -553,10 +553,38 @@ int kvm_vgic_inject_irq(struct kvm *kvm, struct kvm_vcpu *vcpu,
>  	return 0;
>  }
>  
> +void kvm_vgic_set_irq_ops(struct kvm_vcpu *vcpu, u32 vintid,
> +			 struct irq_ops *ops)
> +{
> +	struct vgic_irq *irq = vgic_get_vcpu_irq(vcpu, vintid);
> +
> +	BUG_ON(!irq);
> +
> +	scoped_guard(raw_spinlock_irqsave, &irq->irq_lock)
> +	{
> +		irq->ops = ops;
> +	}

nit: opening brace in the wrong spot, and overall not useful. This
could simply be written as:

	scoped_guard(raw_spinlock_irqsave, &irq->irq_lock)
		irq->ops = ops;

> +
> +	vgic_put_irq(vcpu->kvm, irq);
> +}
> +
> +void kvm_vgic_clear_irq_ops(struct kvm_vcpu *vcpu, u32 vintid)
> +{
> +	struct vgic_irq *irq = vgic_get_vcpu_irq(vcpu, vintid);
> +
> +	BUG_ON(!irq);
> +
> +	scoped_guard(raw_spinlock_irqsave, &irq->irq_lock)
> +	{
> +		irq->ops = NULL;
> +	}
> +
> +	vgic_put_irq(vcpu->kvm, irq);
> +}
> +

nit: that could also be written as:

void kvm_vgic_clear_irq_ops(struct kvm_vcpu *vcpu, u32 vintid)
{
	kvm_vgic_set_irq_ops(vcpu, vintid, NULL);
}

I can fix all of it when applying if that works for you.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.