Re: [PATCH v4 04/11] perf/cxl: Accept an overflow interrupt on MSI message number 0

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Modifying CPU affinity on a shared interrupt line alters the affinity for all devices sharing the line.
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--- Patch [4]: [PATCH v4 04/11] perf/cxl: Accept an overflow interrupt on MSI message number 0 ---
commit 54fa1d2700a4dd3fe1865a2f2f6744ccbafc3070
Author: Dave Jiang <[email protected]>
Subject: perf/cxl: Accept an overflow interrupt on MSI message number 0

cxl_pmu_probe() rejects the PMU when info->irq <= 0, but at that point the
field still holds the MSI/MSI-X message number the device signals overflow
on, not a Linux virq. That number 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.

Link: https://sashiko.dev/#/patchset/[email protected]?part=1

> diff --git a/drivers/perf/cxl_pmu.c b/drivers/perf/cxl_pmu.c
> index 40741e529d9b2..2bebbbe67a0a2 100644
> --- a/drivers/perf/cxl_pmu.c
> +++ b/drivers/perf/cxl_pmu.c
> @@ -873,7 +873,7 @@ static int cxl_pmu_probe(struct device *dev)
>  		.capabilities = PERF_PMU_CAP_NO_EXCLUDE,
>  	};
>  
> -	if (info->irq <= 0)
> +	if (info->irq < 0)
>  		return -EINVAL;
>  
>  	rc = pci_irq_vector(pdev, info->irq);

[Severity: High]
This is a pre-existing issue, but I noticed the interrupt is requested as
shared in cxl_pmu_probe() and then its affinity is forcibly modified during
hotplug operations.

The interrupt is requested as a shared interrupt:

	rc = devm_request_irq(dev, irq, cxl_pmu_irq, IRQF_SHARED | IRQF_NO_THREAD,
			      irq_name, info);

But the CPU affinity is later altered directly in cxl_pmu_online_cpu():

	WARN_ON(irq_set_affinity(info->irq, cpumask_of(cpu)));

And similarly in cxl_pmu_offline_cpu():

	WARN_ON(irq_set_affinity(info->irq, cpumask_of(target)));

Does this cause issues for other devices sharing the interrupt line?

Modifying CPU affinity on a shared interrupt alters the affinity for all devices
sharing the line, which could break CPU affinity expectations for other
endpoints and potentially cause performance degradation or incorrect behavior.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=4