Re: [PATCH v3 03/12] KVM: selftests: Pre-set threads affinity in hardware disable test when possible

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] The fallback logic for setting thread CPU affinity is guarded by `#ifndef _GNU_SOURCE`, which is always false because KVM selftests unconditionally define `_GNU_SOURCE`. This renders the fallback dead code and breaks builds on non-glibc systems.
--

--- Patch [3]: [PATCH v3 03/12] KVM: selftests: Pre-set threads affinity in hardware disable test when possible ---
commit 242db3bb38cdcdb7592d9d34951b2c689c2c39b5
Author: Sean Christopherson <seanjc@google.com>

KVM: selftests: Pre-set threads affinity in hardware disable test when possible

This commit pre-sets thread affinity using pthread_attr_setaffinity_np to
reduce pthread creation latency on large NUMA systems. It attempts to add a
fallback using kvm_sched_setaffinity when _GNU_SOURCE is not defined.

> diff --git a/tools/testing/selftests/kvm/hardware_disable_test.c b/tools/testing/selftests/kvm/hardware_disable_test.c
> index f088b4af3ccc7..b4edeadfa3f45 100644
> --- a/tools/testing/selftests/kvm/hardware_disable_test.c
> +++ b/tools/testing/selftests/kvm/hardware_disable_test.c
> @@ -36,7 +36,9 @@ static void *run_vcpu(void *arg)
>  	struct kvm_vcpu *vcpu = arg;
>  	struct kvm_run *run = vcpu->run;
>  
> +#ifndef _GNU_SOURCE
>  	kvm_sched_setaffinity(0, sizeof(cpu_set_t), &threads_cpu_set);
> +#endif

[Severity: Low]
Since the KVM selftests build system unconditionally defines _GNU_SOURCE,
will this fallback code ever be reached?

[ ... ]

> @@ -80,22 +84,28 @@ static inline void check_join(pthread_t thread, void **retval)
>  static void run_test(u32 run)
>  {
>  	struct kvm_vcpu *vcpu;
> +	pthread_attr_t attr;
>  	struct kvm_vm *vm;
>  	pthread_t threads[VCPU_NUM];
>  	pthread_t throw_away;
>  	void *b;
>  	u32 i, j;
>  
> +	TEST_ASSERT_EQ(pthread_attr_init(&attr), 0);
> +#ifdef _GNU_SOURCE
> +	TEST_ASSERT_EQ(pthread_attr_setaffinity_np(&attr, sizeof(cpu_set_t), &threads_cpu_set), 0);
> +#endif

[Severity: Low]
Could this break the build on systems using alternative libc implementations
like musl?

If _GNU_SOURCE is unconditionally defined by the Makefile but the libc
doesn't provide pthread_attr_setaffinity_np, this might result in a
compilation error rather than using the intended fallback.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260731195612.2697986-1-seanjc@google.com?part=3