Re: [PATCH v2 08/10] KVM: selftests: Trigger save+restore randomly in the #PF stress test

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Aggressive 100us signal interval causes test livelock and spurious failures on slower hosts.
--

commit bf3ad650cf1e2ff2e1e4bd996b52e985a3657486
Author: Yosry Ahmed <yosry@kernel.org>

KVM: selftests: Trigger save+restore randomly in the #PF stress test

This commit updates the #PF stress test to trigger exits and save/restore
operations randomly via a background thread sending SIGUSR1. This replaces
the explicit GUEST_SYNC() calls to better exercise race conditions during
guest execution.

> diff --git a/tools/testing/selftests/kvm/x86/stress_save_restore_pf_test.c b/tools/testing/selftests/kvm/x86/stress_save_restore_pf_test.c
> index 622d102179e66..92efb7ac9d95f 100644
> --- a/tools/testing/selftests/kvm/x86/stress_save_restore_pf_test.c
> +++ b/tools/testing/selftests/kvm/x86/stress_save_restore_pf_test.c

[ ... ]

> @@ -94,15 +96,41 @@ static void guest_access_memory(void *arg)

[ ... ]

> +static void *sigusr_thread_fn(void *arg)
> +{
> +	pthread_t vcpu_thread = (pthread_t)arg;
>  
> -		GUEST_SYNC(guest_faults);
> +	for (;;) {
> +		pthread_testcancel();
> +		pthread_kill(vcpu_thread, SIGUSR1);
> +		usleep(100);

[Severity: Medium]
Could this aggressive 100-microsecond signal interval cause a livelock on
slower hosts or nested environments?

If the KVM VM-entry and EPT/NPT setup takes longer than 100 microseconds,
the pending signal might cause KVM to abort the entry and return -EINTR to
userspace before the guest makes any forward progress.

Because the main loop fully destroys and recreates the VM on every iteration
(via kvm_vm_release() and vm_recreate_with_one_vcpu()), the MMU state is
dropped, meaning the next loop starts completely cold. If every entry takes
longer than 100us, the test will repeatedly fail to enter the guest.

>  	}
> +	return NULL;
> +}

[ ... ]

> @@ -175,8 +215,11 @@ int main(int argc, char *argv[])
>  	}
>  
>  	sync_global_from_guest(vm, guest_faults);
> +	TEST_ASSERT(guest_faults > 0, "No guest page faults triggered");

[Severity: Medium]
If the VM is livelocked as described above, will this assertion spuriously
fail because the guest never executed an instruction?

>  	pr_info("Guest page faults: %lu\n", guest_faults);
>  
> +	pthread_cancel(sigusr_thread);
> +	pthread_join(sigusr_thread, NULL);
>  	kvm_vm_free(vm);
>  	return 0;
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260604203546.365658-1-yosry@kernel.org?part=8