[PATCH kernel v2] powerpc/xive: Drop deregistered irqs

[Cédric Le Goater]

On 15/07/2019 09:11, 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 adds a new ppc_md.orphan_irq callback which is called if no irq
> descriptor is found. The XIVE implementation drops the current priority
> to 0xff which effectively unmasks interrupts in a current CPU.


The test on generic_handle_irq() catches interrupt events that
were served on a target CPU while the source interrupt was being
shutdown on another CPU.

The orphan_irq() handler restores the CPPR in such cases. 

This looks OK to me. I would have added some more comments in the 
code. 

Reviewed-by: Cédric Le Goater <clg at kaod.org>


And adding to the list of future cleanups : a 'set_cppr' helper.

Thanks,

C.


> Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>
> ---
> Changes:
> v2:
> * added ppc_md.orphan_irq
> 
> ---
> 
> 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.
> ---
>  arch/powerpc/include/asm/machdep.h |  3 +++
>  arch/powerpc/kernel/irq.c          |  9 ++++++---
>  arch/powerpc/sysdev/xive/common.c  | 10 ++++++++++
>  3 files changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/machdep.h b/arch/powerpc/include/asm/machdep.h
> index c43d6eca9edd..6cc14e28e89a 100644
> --- a/arch/powerpc/include/asm/machdep.h
> +++ b/arch/powerpc/include/asm/machdep.h
> @@ -59,6 +59,9 @@ struct machdep_calls {
>  	/* Return an irq, or 0 to indicate there are none pending. */
>  	unsigned int	(*get_irq)(void);
>  
> +	/* Drops irq if it does not have a valid descriptor */
> +	void		(*orphan_irq)(unsigned int irq);
> +
>  	/* PCI stuff */
>  	/* Called after allocating resources */
>  	void		(*pcibios_fixup)(void);
> diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
> index bc68c53af67c..b4e06d05bdba 100644
> --- a/arch/powerpc/kernel/irq.c
> +++ b/arch/powerpc/kernel/irq.c
> @@ -632,10 +632,13 @@ void __do_irq(struct pt_regs *regs)
>  	may_hard_irq_enable();
>  
>  	/* And finally process it */
> -	if (unlikely(!irq))
> +	if (unlikely(!irq)) {
>  		__this_cpu_inc(irq_stat.spurious_irqs);
> -	else
> -		generic_handle_irq(irq);
> +	} else if (generic_handle_irq(irq)) {
> +		if (ppc_md.orphan_irq)
> +			ppc_md.orphan_irq(irq);
> +		__this_cpu_inc(irq_stat.spurious_irqs);
> +	}
>  
>  	trace_irq_exit(regs);
>  
> diff --git a/arch/powerpc/sysdev/xive/common.c b/arch/powerpc/sysdev/xive/common.c
> index 082c7e1c20f0..b4054091999a 100644
> --- a/arch/powerpc/sysdev/xive/common.c
> +++ b/arch/powerpc/sysdev/xive/common.c
> @@ -283,6 +283,15 @@ static unsigned int xive_get_irq(void)
>  	return irq;
>  }
>  
> +static void xive_orphan_irq(unsigned int irq)
> +{
> +	struct xive_cpu *xc = __this_cpu_read(xive_cpu);
> +
> +	xc->cppr = 0xff;
> +	out_8(xive_tima + xive_tima_offset + TM_CPPR, 0xff);
> +	DBG_VERBOSE("orphan_irq: irq %d, adjusting CPPR to 0xff\n", irq);
> +}
> +
>  /*
>   * After EOI'ing an interrupt, we need to re-check the queue
>   * to see if another interrupt is pending since multiple
> @@ -1419,6 +1428,7 @@ bool __init xive_core_init(const struct xive_ops *ops, void __iomem *area, u32 o
>  	xive_irq_priority = max_prio;
>  
>  	ppc_md.get_irq = xive_get_irq;
> +	ppc_md.orphan_irq = xive_orphan_irq;
>  	__xive_enabled = true;
>  
>  	pr_devel("Initializing host..\n");
>