Re: [PATCH v3 5/9] perf/cxl: Accept an overflow interrupt on MSI message number 0

[Richard Cheng <icheng@nvidia.com>]

On Fri, Jul 31, 2026 at 04:28:23PM +0800, Dave Jiang wrote:
> cxl_pmu_probe() rejects the PMU when info->msi_vec <= 0, but that field
> holds the MSI/MSI-X message number the device signals overflow on. The
> field is 0-based, and -1 means no interrupt support. Message number 0 is
> valid and pci_irq_vector() takes a 0-based index, so a compliant device
> signalling on the first vector fails to probe.
> 
> Reject only the no-interrupt case, matching how the CXL mailbox and event
> interrupts handle it.
> 
> Fixes: 5d7107c72796 ("perf: CXL Performance Monitoring Unit driver")
> Reported-by: sashiko-bot@kernel.org
> Closes: https://sashiko.dev/#/patchset/20260715191454.459673-1-dave@stgolabs.net?part=1
> Assisted-by: Claude:claude-opus-4-8
> Reviewed-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>
> Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> ---
> v3:
> - Rebase on the preceding patch, so this tests info->msi_vec rather than
>   info->irq. The fix is unchanged and the hunk is still one line.
> - Not guarding against pci_irq_vector()'s INTx fallback for vector 0, which
>   it returns when neither MSI nor MSI-X is enabled. A v3 draft added a
>   pci_dev_msi_enabled() check for that, but a modern CXL device wiring up
>   INTx is not a realistic prospect, and the consequence otherwise is a PMU
>   that counts nothing rather than anything unsafe. Left alone to keep this
>   a one-liner.
> ---
>  drivers/perf/cxl_pmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/perf/cxl_pmu.c b/drivers/perf/cxl_pmu.c
> index 56e6ae7bebf7..c9e30cb149df 100644
> --- a/drivers/perf/cxl_pmu.c
> +++ b/drivers/perf/cxl_pmu.c
> @@ -875,7 +875,7 @@ static int cxl_pmu_probe(struct device *dev)
>  		.capabilities = PERF_PMU_CAP_NO_EXCLUDE,
>  	};
>  
> -	if (info->msi_vec <= 0)
> +	if (info->msi_vec < 0)
>  		return -EINVAL;
>

Hi Dave,

I agree that 0 is a legal message number, but do we need to check whether
MSI is actually enabled ?

something like
"""
if (info->msi_vec < 0 || (!pdev->msi_enabled && !pdev->msix_enabled))
    return -EINVAL;
"""

Best regards,
Richard Cheng




>  	rc = pci_irq_vector(pdev, info->msi_vec);
> -- 
> 2.55.0
> 
>