Re: [PATCH v3 2/4] KVM: selftests: add is_cpu_online() utility function

[Sean Christopherson <seanjc@google.com>]

On Tue, Mar 22, 2022, Ricardo Koller wrote:
> Add is_cpu_online() utility function: a wrapper for
> "/sys/devices/system/cpu/cpu%d/online".
> 
> Signed-off-by: Ricardo Koller <ricarkol@google.com>
> ---
>  tools/testing/selftests/kvm/include/test_util.h |  2 ++
>  tools/testing/selftests/kvm/lib/test_util.c     | 16 ++++++++++++++++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/tools/testing/selftests/kvm/include/test_util.h b/tools/testing/selftests/kvm/include/test_util.h
> index 99e0dcdc923f..14084dc4e152 100644
> --- a/tools/testing/selftests/kvm/include/test_util.h
> +++ b/tools/testing/selftests/kvm/include/test_util.h
> @@ -143,4 +143,6 @@ static inline void *align_ptr_up(void *x, size_t size)
>  	return (void *)align_up((unsigned long)x, size);
>  }
>  
> +bool is_cpu_online(int pcpu);
> +
>  #endif /* SELFTEST_KVM_TEST_UTIL_H */
> diff --git a/tools/testing/selftests/kvm/lib/test_util.c b/tools/testing/selftests/kvm/lib/test_util.c
> index 6d23878bbfe1..81950e6b6d10 100644
> --- a/tools/testing/selftests/kvm/lib/test_util.c
> +++ b/tools/testing/selftests/kvm/lib/test_util.c
> @@ -334,3 +334,19 @@ long get_run_delay(void)
>  
>  	return val[1];
>  }
> +
> +bool is_cpu_online(int pcpu)
> +{
> +	char p[128];
> +	FILE *fp;
> +	int ret;
> +
> +	snprintf(p, sizeof(p), "/sys/devices/system/cpu/cpu%d/online", pcpu);

I don't think this is sufficient for the use in patch 03; the CPU could be online
but disallowed for use by the current task.  I think what you want instead is a
combination of get_nprocs_conf() + sched_getaffinity() + CPU_ISSET().