Re: [PATCH v5 07/21] KVM: selftests: Verify IRQ bypass works in IRQ test

[Sean Christopherson <seanjc@google.com>]

The shortlog is at best misleading.  There are zero guarantees that IRQ bypass
is supported and enabled.

On Thu, Jun 04, 2026, Josh Hilke wrote:
> From: David Matlack <dmatlack@google.com>
> 
> Trigger interrupts from a VFIO device instead of emulating interrupts

It's not emulating, it's synthesizing.  Emulating implies there's a device of
some kind that the test is mimicking.

> using KVM eventfds. This verifies that guests receive interrupts via IRQ
> bypass.

No, it verifies that delivery of a VFIO MSI-X through VFIO=>KVM works, and *may*
verify IRQ bypass.  And all of that very much relies on the eventfds to be in
place.

> @@ -119,7 +160,17 @@ int main(int argc, char **argv)
>  	vm = vm_create_with_vcpus(nr_vcpus, guest_code, vcpus);
>  	vm_install_exception_handler(vm, vector, guest_irq_handler);
>  
> -	eventfd = kvm_new_eventfd();
> +	if (device_bdf) {
> +		iommu = iommu_init(default_iommu_mode);

This needs:

diff --git tools/testing/selftests/kvm/irq_test.c tools/testing/selftests/kvm/irq_test.c
index cf4568718cee..2e7e100d4815 100644
--- tools/testing/selftests/kvm/irq_test.c
+++ tools/testing/selftests/kvm/irq_test.c
@@ -235,6 +235,8 @@ int main(int argc, char **argv)
        }
 
        if (device_bdf) {
+               __open_path_or_exit("/dev/iommu", O_RDONLY, "Is IOMMUFD available?");
+
                iommu = iommu_init(default_iommu_mode);
                device = vfio_pci_device_init(device_bdf, iommu);
                msix = vfio_setup_msi(device);

So that the test skips instead of failing, and to provide a helpful message:

  1..0 # SKIP - Cannot open '/dev/iommu': No such file or directory.  Is IOMMUFD available?

Because bluntly, the assert from the VFIO infrastructure is awful:

  tools/testing/selftests/vfio/lib/iommu.c:448: Assertion Failure

  Expression: iommu->iommufd > 0
  Observed: 0xffffffffffffffff > 0
  [errno: 2 - No such file or directory]