Re: [RFC PATCH] arm_mpam: remove sanity check of accessibility when error interrupt is SPI on SMT platforms

[Ben Horgan]

Hi Yadong,

On 7/20/26 08:24, Yadong Qi wrote:
> On SMT platforms, an L2 cache MSC is typically shared between sibling
> threads. Per the MPAM ACPI spec (DEN0065B), the MSC's linked device
> should be set to the processor container of those siblings, so the
> kernel derives msc->accessibility as just the sibling CPUs.
> 
> Per the MPAM spec (IHI0099B), when an MSC is not integrated into a PE,
> SPI/LPI is the recommended error interrupt. SPIs are not per-CPU, and a
> sanity check in mpam_msc_setup_error_irq() requires accessibility ==
> cpu_possible_mask — assuming a shared interrupt must be routable to all
> CPUs. This creates a conflict on SMT platforms: the MSC has a restricted
> accessibility but a shared error interrupt, causing probe to fail with:
> 
>   msc:N is a private resource with a shared error interrupt
> 
> This RFC patch removes the check to allow MSC probe to succeed on SMT
> platforms, but this is mainly intended to start a discussion. We would
> appreciate advice on the right way to handle this conflict. Is this a
> spec issue, or should the driver handle SPI error interrupts differently

For advice on questions broader than just linux please ask arm support: support@xxxxxxx

> when an MSC has a restricted accessibility mask?

I expect the MPAM driver could be taught to deal with SPI interrupts without global affinity.

> 
> Signed-off-by: Yadong Qi <yadong.qi@xxxxxxxxxxxxxxxxx>
> ---
>  drivers/resctrl/mpam_devices.c | 7 -------
>  1 file changed, 7 deletions(-)
> 
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index b69f99488111..e8b2b5e00d7c 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -1964,13 +1964,6 @@ static int mpam_msc_setup_error_irq(struct mpam_msc *msc)
>  	if (irq_is_percpu(irq))
>  		return __setup_ppi(msc);
>  
> -	/* sanity check: shared interrupts can be routed anywhere? */
> -	if (!cpumask_equal(&msc->accessibility, cpu_possible_mask)) {
> -		pr_err_once("msc:%u is a private resource with a shared error interrupt",
> -			    msc->id);
> -		return -EINVAL;
> -	}
> -

To do this the MPAM driver would have to remove the interrupt from balancing, IRQF_NOBALANCING, and
control the balancing itself.

Thanks,

Ben

>  	return 0;
>  }
>