This patch fixed an issue I was experiencing with virsh start/destroy
of guests with mlx5 and GPU passthrough in a Power 9 server. I
believe it's a similar situation which Alexey described in the post
commit msg.


Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>


On 7/12/19 5:20 AM, Alexey Kardashevskiy wrote:
> There is a race between releasing an irq on one cpu and fetching it
> from XIVE on another cpu as there does not seem to be any locking between
> these, probably because xive_irq_chip::irq_shutdown() is supposed to
> remove the irq from all queues in the system which it does not do.
>
> As a result, when such released irq appears in a queue, we take it
> from the queue but we do not change the current priority on that cpu and
> since there is no handler for the irq, EOI is never called and the cpu
> current priority remains elevated (7 vs. 0xff==unmasked). If another irq
> is assigned to the same cpu, then that device stops working until irq
> is moved to another cpu or the device is reset.
>
> This checks if irq is still registered, if not, it assumes no valid irq
> was fetched from the loop and if there is none left, it continues to
> the irq==0 case (not visible in this patch) and sets priority to 0xff
> which is basically unmasking. This calls irq_to_desc() on a hot path now
> which is a radix tree lookup; hopefully this won't be noticeable as
> that tree is quite small.
>
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> ---
>
> Found it on P9 system with:
> - a host with 8 cpus online
> - a boot disk on ahci with its msix on cpu#0
> - a guest with 2xGPUs + 6xNVLink + 4 cpus
> - GPU#0 from the guest is bound to the same cpu#0.
>
> Killing a guest killed ahci and therefore the host because of the race.
> Note that VFIO masks interrupts first and only then resets the device.
>
> Alternatives:
>
> 1. Fix xive_irq_chip::irq_shutdown() to walk through all cpu queues and
> drop deregistered irqs.
>
> 2. Exploit chip->irq_get_irqchip_state function from
> 62e0468650c30f0298 "genirq: Add optional hardware synchronization for shutdown".
>
> Both require deep XIVE knowledge which I do not have.
> ---
>   arch/powerpc/sysdev/xive/common.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/sysdev/xive/common.c b/arch/powerpc/sysdev/xive/common.c
> index 082c7e1c20f0..65742e280337 100644
> --- a/arch/powerpc/sysdev/xive/common.c
> +++ b/arch/powerpc/sysdev/xive/common.c
> @@ -148,8 +148,12 @@ static u32 xive_scan_interrupts(struct xive_cpu *xc, bool just_peek)
>   		irq = xive_read_eq(&xc->queue[prio], just_peek);
>   
>   		/* Found something ? That's it */
> -		if (irq)
> -			break;
> +		if (irq) {
> +			/* Another CPU may have shut this irq down, check it */
> +			if (irq_to_desc(irq))
> +				break;
> +			irq = 0;
> +		}
>   
>   		/* Clear pending bits */
>   		xc->pending_prio &= ~(1 << prio);