Re: [PATCH v6 09/10] arm_mpam: change MPAM-Fb error IRQ to use a threaded IRQ handler

[Ben Horgan <ben.horgan@arm.com>]

Hi Andre,

On 7/30/26 16:25, Andre Przywara wrote:
> When an MPAM MSC gets into an error condition, it can trigger an error
> IRQ. We cannot really do much about those errors, but we at least query
> and log the error, then disable MPAM functionality.
> 
> This error report relies on reading the MSC's error status register
> (ESR) in the current hard-IRQ handler, which is not possible for MPAM-Fb
> based MSC accesses, since they involve mailbox routines that might sleep.
> The same is true for clearing the interrupt at the source, which requires
> an MSC access as well.
> 
> When an MSC is using MPAM-Fb, change the error IRQ to use a threaded IRQ
> handler, with an empty hard IRQ routine, and doing all the MSC accesses
> (to access the status and disable the IRQ line) in the threaded part.
> The change is minimal, we just check for the first MSC access error and
> bail out early. MMIO based MSCs keep using a hard-IRQ handler, since
> they must be at least non-migrate-able when doing MSC accesses, for the
> CPU affinity check to work.
> Also forbid per-CPU interrupts (PPIs) for MPAM-Fb, as we cannot use a
> threaded IRQ here.
> 
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> ---
>  drivers/resctrl/mpam_devices.c | 64 ++++++++++++++++++++++++++--------
>  1 file changed, 49 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index daaf3cf48475..606ae4af2be9 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -2660,24 +2660,38 @@ static int mpam_disable_msc_ecr(void *_msc)
>  	return 0;
>  }
>  
> +/*
> + * This will run as the threaded IRQ handler part when using MPAM-Fb, but
> + * as the sole hard-IRQ handler for MMIO based accesses.
> + */
>  static irqreturn_t __mpam_irq_handler(int irq, struct mpam_msc *msc)
>  {
>  	u64 reg;
> +	int ret;
>  	u16 partid;
>  	u8 errcode, pmg, ris;
>  
> -	if (WARN_ON_ONCE(!msc) ||
> +	if (WARN_ON_ONCE(!msc))
> +		return IRQ_NONE;
> +
> +	if (msc->iface == MPAM_IFACE_MMIO &&
>  	    WARN_ON_ONCE(!cpumask_test_cpu(smp_processor_id(),
>  					   &msc->accessibility)))
>  		return IRQ_NONE;
>  
> -	mpam_msc_read_esr(msc, &reg);
> +	ret = mpam_msc_read_esr(msc, &reg);
> +	if (ret) {
> +		pr_err_ratelimited("unknown error irq from msc:%u\n", msc->id);
> +
> +		/* Try out best here ... */
> +		goto out_disable;
> +	}
>  
>  	errcode = FIELD_GET(MPAMF_ESR_ERRCODE, reg);
>  	if (!errcode)
>  		return IRQ_NONE;
>  
> -	/* Clear level triggered irq */
> +	/* Clear level triggered irq. Ignore errors, we need to proceed. */
>  	mpam_msc_clear_esr(msc);
>  
>  	partid = FIELD_GET(MPAMF_ESR_PARTID_MON, reg);
> @@ -2688,19 +2702,19 @@ static irqreturn_t __mpam_irq_handler(int irq, struct mpam_msc *msc)
>  			   msc->id, mpam_errcode_names[errcode], partid, pmg,
>  			   ris);
>  
> -	/* Disable this interrupt. */
> +out_disable:
> +	/* Disable this interrupt. Ignore errors, we need to proceed anyway. */
>  	mpam_disable_msc_ecr(msc);

If we fail to disable the interrupt then I don't think we can claim we handled it.
Shouldn't we, after scheduling broken work, return IRQ_NONE?

Thanks,

Ben

>  
> -	/* Are we racing with the thread disabling MPAM? */
> -	if (!mpam_is_enabled())
> -		return IRQ_HANDLED;
> -
>  	/*
> -	 * Schedule the teardown work. Don't use a threaded IRQ as we can't
> -	 * unregister the interrupt from the threaded part of the handler.
> +	 * Schedule the teardown work. We have to defer it as we can't
> +	 * unregister the interrupt from the threaded part of a handler.
> +	 * Check whether we are racing with the thread disabling MPAM.
>  	 */
> -	mpam_disable_reason = "hardware error interrupt";
> -	schedule_work(&mpam_broken_work);
> +	if (mpam_is_enabled()) {
> +		mpam_disable_reason = "hardware error interrupt";
> +		schedule_work(&mpam_broken_work);
> +	}
>  
>  	return IRQ_HANDLED;
>  }
> @@ -2736,6 +2750,11 @@ static int mpam_register_irqs(void)
>  		/* The MPAM spec says the interrupt can be SPI, PPI or LPI */
>  		/* We anticipate sharing the interrupt with other MSCs */
>  		if (irq_is_percpu(irq)) {
> +			if (msc->iface != MPAM_IFACE_MMIO) {
> +				dev_err(&msc->pdev->dev,
> +					"Only MMIO MSCs can use per-CPU interrupts\n");
> +				return -EINVAL;
> +			}
>  			err = request_percpu_irq(irq, &mpam_ppi_handler,
>  						 "mpam:msc:error",
>  						 msc->error_dev_id);
> @@ -2747,9 +2766,24 @@ static int mpam_register_irqs(void)
>  					       &_enable_percpu_irq, &irq,
>  					       true);
>  		} else {
> -			err = devm_request_irq(&msc->pdev->dev, irq,
> -					       &mpam_spi_handler, IRQF_SHARED,
> -					       "mpam:msc:error", msc);
> +			irq_handler_t hard, threaded;
> +
> +			/*
> +			 * MPAM-Fb needs a sleepable context for the MSC
> +			 * accesses, whereas MMIO requires at least a
> +			 * non-migrateable context for the processor ID check.
> +			 */
> +			if (msc->iface == MPAM_IFACE_MMIO) {
> +				hard = &mpam_spi_handler;
> +				threaded = NULL;
> +			} else {
> +				hard = NULL;
> +				threaded = &mpam_spi_handler;
> +			}
> +			err = devm_request_threaded_irq(&msc->pdev->dev, irq,
> +							hard, threaded,
> +							IRQF_SHARED | IRQF_ONESHOT,
> +							"mpam:msc:error", msc);
>  			if (err)
>  				return err;
>  		}