Re: [PATCH v5 09/10] arm_mpam: change error IRQ to use a threaded IRQ handler

[Ben Horgan]

Hi Andre,

On 7/29/26 14:41, 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 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.
> 
> 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.
> Also forbid per-CPU interrupts (PPIs) for MPAM-Fb, as we cannot use a
> threaded IRQ here.
> 
> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
> ---
>  drivers/resctrl/mpam_devices.c | 42 ++++++++++++++++++++++------------
>  1 file changed, 28 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index abe1e628928f..e535603d2c7d 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -2659,9 +2659,11 @@ static int mpam_disable_msc_ecr(void *_msc)
>  	return 0;
>  }
>  
> +/* threaded IRQ handler for shared IRQs, to allow MPAM-Fb accesses to sleep */

This comment seems incomplete. The same handler is used for the PPI hardirq.
>  static irqreturn_t __mpam_irq_handler(int irq, struct mpam_msc *msc)
>  {
>  	u64 reg;
> +	int ret;
>  	u16 partid;
>  	u8 errcode, pmg, ris;
>  
> @@ -2670,13 +2672,19 @@ static irqreturn_t __mpam_irq_handler(int irq, struct mpam_msc *msc)
>  					   &msc->accessibility)))
>  		return IRQ_NONE;
>  
> -	mpam_msc_read_esr(msc, &reg);
> +	ret = mpam_msc_read_esr(msc, &reg);

This is also called for MMIO accesses and the checks in the reads and writes call smp_processor_id()
which will give warnings if we are preemptible. There is also a similar to check higher up in this
handler. We need to choose in what context we call things based on whether we are using MMIO
accesses or MPAM-Fb.

Thanks,

Ben

> +	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);
> @@ -2687,19 +2695,19 @@ 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);
>  
> -	/* 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;
>  }
> @@ -2735,6 +2743,11 @@ 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);
> @@ -2746,9 +2759,10 @@ 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);
> +			err = devm_request_threaded_irq(&msc->pdev->dev, irq,
> +							NULL, &mpam_spi_handler,
> +							IRQF_SHARED | IRQF_ONESHOT,
> +							"mpam:msc:error", msc);
>  			if (err)
>  				return err;
>  		}