Re: [PATCH v3 13/16] KVM: selftests: Verify vCPU migration during IRQ delivery

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 21, 2026, Josh Hilke wrote:
> From: David Matlack <dmatlack@google.com>
> 
> Add the '-m' flag to irq_test to migrate vCPU threads across random
> physical CPUs during the test. This validates KVM's ability to handle
> vCPUs changing physical CPUs while interrupts are actively being
> injected.
> 
> To support this, add wrappers for gettid() and sched_getaffinity(), and
> introduce pin_task_to_random_cpu() in lib/kvm_util.c.
> 
> Co-developed-by: Josh Hilke <jrhilke@google.com>
> Signed-off-by: Josh Hilke <jrhilke@google.com>
> Signed-off-by: David Matlack <dmatlack@google.com>
> ---
>  .../selftests/kvm/include/kvm_syscalls.h      |  6 ++++
>  .../testing/selftests/kvm/include/kvm_util.h  |  2 ++
>  tools/testing/selftests/kvm/irq_test.c        | 34 +++++++++++++++++--
>  tools/testing/selftests/kvm/lib/kvm_util.c    | 19 +++++++++++
>  4 files changed, 59 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/include/kvm_syscalls.h b/tools/testing/selftests/kvm/include/kvm_syscalls.h
> index d4e613162bba..d73f45c5df92 100644
> --- a/tools/testing/selftests/kvm/include/kvm_syscalls.h
> +++ b/tools/testing/selftests/kvm/include/kvm_syscalls.h
> @@ -73,9 +73,15 @@ static inline int kvm_dup(int fd)
>  	return new_fd;
>  }
>  
> +static inline int gettid(void)

gettid() already exists in at least some of my environments:

include/kvm_syscalls.h:87:19: error: static declaration of ‘gettid’ follows non-static declaration
   87 | static inline int gettid(void)
      |                   ^~~~~~
In file included from /usr/include/unistd.h:1221,
                 from /usr/include/x86_64-linux-gnu/bits/sigstksz.h:24,
                 from /usr/include/signal.h:328,
                 from include/test_util.h:12,
                 from lib/kvm_util.c:7:
/usr/include/x86_64-linux-gnu/bits/unistd_ext.h:34:16: note: previous declaration of ‘gettid’ with type ‘__pid_t(void)’ {aka ‘int(void)’}
   34 | extern __pid_t gettid (void) __THROW;
      |                ^~~~~~

> +void pin_task_to_random_cpu(pthread_t task, cpu_set_t *possible_cpus)
> +{
> +	int i = 0, nr_cpus = CPU_COUNT(possible_cpus);
> +	int cpu, target_idx;
> +
> +	target_idx = kvm_random_u64(&kvm_rng) % nr_cpus;
> +
> +	for (cpu = 0; i < nr_cpus; cpu++) {

Shouldn't this be bounded on "cpu < nr_cpus"?  If 'i' gets to nr_cpus, it means
"i == target_idx" was never taken.  And at that point, the loop will hang
indefinitely because cpu will likely be waaaay out of bounds of possible_cpus,
i.e. the loop will always continue.

> +		if (!CPU_ISSET(cpu, possible_cpus))
> +			continue;
> +
> +		if (i == target_idx) {
> +			pin_task_to_cpu(task, cpu);
> +			return;
> +		}
> +		i++;
> +	}

This should TEST_FAIL().  Or maybe "break" on the match, and the assert that
i == target_idx?

> +}
> +
>  void kvm_parse_vcpu_pinning(const char *pcpus_string, uint32_t vcpu_to_pcpu[],
>  			    int nr_vcpus)
>  {
> -- 
> 2.54.0.rc2.533.g4f5dca5207-goog
>