Re: [PATCH] x86/events/amd/iommu: Fix cpumask of IOMMU events

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] PMU initialization fails for IOMMUs located on NUMA nodes without online CPUs.
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commit ef1363d0a37870d7cf1f1095cd3ebf42cc5a8bdc
Author: Vasant Hegde <vasant.hegde@amd.com>

x86/events/amd/iommu: Fix cpumask of IOMMU events

This commit changes the IOMMU PMU cpumask from a single global mask to a
per-IOMMU mask based on the NUMA node of the IOMMU, allowing per-socket
performance counter collection.

> diff --git a/arch/x86/events/amd/iommu.c b/arch/x86/events/amd/iommu.c
> --- a/arch/x86/events/amd/iommu.c
> +++ b/arch/x86/events/amd/iommu.c
[ ... ]
> @@ -440,6 +445,20 @@ static __init int init_one_iommu(unsigned int idx)
>  		return -EINVAL;
>  	}
>  
> +	dev = amd_iommu_idx_to_dev(idx);
> +	node = dev ? dev_to_node(dev) : NUMA_NO_NODE;
> +	if (node != NUMA_NO_NODE)
> +		cpu = cpumask_any_and(cpumask_of_node(node), cpu_online_mask);

If the IOMMU resides on a CPU-less NUMA node (like a PCIe expansion switch
or a memory-only CXL node), or if all CPUs on that node are currently
offline, will cpumask_any_and() return a value greater than or equal to
nr_cpu_ids?

> +	else
> +		cpu = cpumask_any(cpu_online_mask);
> +
> +	if (cpu >= nr_cpu_ids) {
> +		pr_warn("Failed to find online CPU for IOMMU %d.\n", idx);
> +		kfree(perf_iommu);
> +		return -ENODEV;
> +	}

Could this cause PMU initialization to fail for IOMMUs on CPU-less nodes?

Since AMD IOMMU performance counters are accessed via MMIO space, they can
be driven by any CPU in the system. Would it be better to fall back to
cpumask_any(cpu_online_mask) instead of returning -ENODEV when a local CPU
is not available?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260517114026.6817-1-vasant.hegde@amd.com?part=1