Re: [PATCH] xen/events: Fix Global and Domain VIRQ tracking

[Jason Andryuk <jason.andryuk@amd.com>]

On 2025-08-12 15:00, Jason Andryuk wrote:
> VIRQs come in 3 flavors, per-VPU, per-domain, and global.  The existing
> tracking of VIRQs is handled by per-cpu variables virq_to_irq.
> 
> The issue is that bind_virq_to_irq() sets the per_cpu virq_to_irq at
> registration time - typically CPU 0.  Later, the interrupt can migrate,
> and info->cpu is updated.  When calling unbind_from_irq(), the per-cpu
> virq_to_irq is cleared for a different cpu.  If bind_virq_to_irq() is
> called again with CPU 0, the stale irq is returned.
> 
> Change the virq_to_irq tracking to use CPU 0 for per-domain and global
> VIRQs.  As there can be at most one of each, there is no need for
> per-vcpu tracking.  Also, per-domain and global VIRQs need to be
> registered on CPU 0 and can later move, so this matches the expectation.
> 
> Fixes: e46cdb66c8fc ("xen: event channels")
> Cc: stable@vger.kernel.org
> Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
> ---
> Fixes is the introduction of the virq_to_irq per-cpu array.
> 
> This was found with the out-of-tree argo driver during suspend/resume.
> On suspend, the per-domain VIRQ_ARGO is unbound.  On resume, the driver
> attempts to bind VIRQ_ARGO.  The stale irq is returned, but the
> WARN_ON(info == NULL || info->type != IRQT_VIRQ) in bind_virq_to_irq()
> triggers for NULL info.  The bind fails and execution continues with the
> driver trying to clean up by unbinding.  This eventually faults over the
> NULL info.
> ---
>   drivers/xen/events/events_base.c | 17 ++++++++++++++++-
>   1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/xen/events/events_base.c b/drivers/xen/events/events_base.c
> index 41309d38f78c..a27e4d7f061e 100644
> --- a/drivers/xen/events/events_base.c
> +++ b/drivers/xen/events/events_base.c
> @@ -159,7 +159,19 @@ static DEFINE_MUTEX(irq_mapping_update_lock);
>   
>   static LIST_HEAD(xen_irq_list_head);
>   
> -/* IRQ <-> VIRQ mapping. */
> +static bool is_per_vcpu_virq(int virq) {
> +	switch (virq) {
> +	case VIRQ_TIMER:
> +	case VIRQ_DEBUG:
> +	case VIRQ_XENOPROF:
> +	case VIRQ_XENPMU:
> +		return true;
> +	default:
> +		return false;
> +	}
> +}
> +
> +/* IRQ <-> VIRQ mapping.  Global/Domain virqs are tracked in cpu 0.  */
>   static DEFINE_PER_CPU(int [NR_VIRQS], virq_to_irq) = {[0 ... NR_VIRQS-1] = -1};
>   
>   /* IRQ <-> IPI mapping */
> @@ -974,6 +986,9 @@ static void __unbind_from_irq(struct irq_info *info, unsigned int irq)
>   
>   		switch (info->type) {
>   		case IRQT_VIRQ:
> +			if (!is_per_vcpu_virq(virq_from_irq(info)))
> +				cpu = 0;
> +
>   			per_cpu(virq_to_irq, cpu)[virq_from_irq(info)] = -1;
>   			break;
>   		case IRQT_IPI:

Thinking about it a little more, bind_virq_to_irq() should ensure cpu == 
0 for per-domain and global VIRQs to ensure the property holds.  Also 
virq_to_irq accesses should go through wrappers to ensure all accesses 
are handled consistently.

I'll send a v2.

Regards,
Jason