Re: [PATCH v5 19/21] KVM: selftests: Print vCPU affinity on timeout during IRQ test

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] kvm_print_vcpu_affinity() uses get_nprocs() as the loop boundary, which silently truncates CPUs with indices >= get_nprocs() on sparse topologies.
--

commit 9e2355f47489ee5cf4674888f5c4b7d1058bc869
Author: Josh Hilke <jrhilke@google.com>

KVM: selftests: Print vCPU affinity on timeout during IRQ test

Adds a helper function to print the physical CPU affinity of a vCPU thread 
in a human-readable format. Uses this new helper in the irq_test to provide 
context when a vCPU times out waiting for an interrupt during migration.

> diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> index e538e6147ec3f..7fc3aaf763797 100644
> --- a/tools/testing/selftests/kvm/lib/kvm_util.c
> +++ b/tools/testing/selftests/kvm/lib/kvm_util.c

[ ... ]

> @@ -662,6 +663,54 @@ void kvm_print_vcpu_pinning_help(void)
>  	       "     (default: no pinning)\n", name, name);
>  }
>  
> +void kvm_print_vcpu_affinity(struct kvm_vcpu *vcpu, pid_t tid)
> +{
> +	int nprocs = get_nprocs();
> +	bool first_range = true;
> +	int i, start = -1;
> +	cpu_set_t cpus;
> +
> +	kvm_sched_getaffinity(tid, sizeof(cpus), &cpus);
> +
> +	/*
> +	 * Output format examples:
> +	 * - Single CPU:  "vCPU 0 (TID 1234) affined to pCPU(s): 2"
> +	 * - List:        "vCPU 0 (TID 1234) affined to pCPU(s): 0,2,4"
> +	 * - Range:       "vCPU 0 (TID 1234) affined to pCPU(s): 0-7"
> +	 * - Mixed:       "vCPU 0 (TID 1234) affined to pCPU(s): 0-3,8,10-12"
> +	 */
> +	printf("vCPU %u (TID %d) affined to pCPU(s): ", vcpu->id, tid);
> +
> +	for (i = 0; i <= nprocs; i++) {
> +		/*
> +		 * Iterate to nprocs (inclusive) to ensure that if the last pCPU
> +		 * is part of a range, the 'else' block triggers one final time
> +		 * to flush that range to stdout.
> +		 */
> +		if (i < nprocs && CPU_ISSET(i, &cpus)) {

[Severity: Medium]
Does using get_nprocs() as the loop boundary risk silently truncating CPUs 
on sparse topologies?

get_nprocs() returns the count of currently online CPUs, not the maximum 
CPU ID. If a system has 4 online CPUs but their IDs are 0, 1, 2, and 7, 
get_nprocs() will return 4.

This would cause the loop to terminate prematurely, completely ignoring 
valid but higher-numbered CPU indices (like CPU 7) in the cpu_set_t mask.

Should this iterate up to CPU_SETSIZE instead to encompass the entire mask 
capacity, similar to other helper functions like pin_task_to_random_cpu()?

> +			if (start == -1)
> +				start = i;
> +			continue;
> +		}
> +
> +		if (start != -1) {
> +			int end = i - 1;
> +
> +			if (!first_range)
> +				printf(",");
> +
> +			if (start == end)
> +				printf("%d", start);
> +			else
> +				printf("%d-%d", start, end);
> +
> +			start = -1;
> +			first_range = false;
> +		}
> +	}
> +	printf("\n");
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260604020143.748245-1-jrhilke@google.com?part=19