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

[Sean Christopherson <seanjc@google.com>]

+Aqib

On Fri, Jul 31, 2026, sashiko-bot@kernel.org wrote:
> > 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

Aqib,

Do you happen to definitively know which is the "right" macro to check to detect
support for pthread_attr_setaffinity_np()?  Belatedly, I saw that you used __GLIBC__
in a patch that ultimately wasn't applied[*].

I initially used __USE_GNU here, but then Sashiko yelled at me, so I switched to
_GNU_SOURCE based on Sashiko's suggestion, and now it's yelling at me again :-)


[*] https://lore.kernel.org/all/20250829142556.72577-4-aqibaf@amazon.com

> [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