Re: [PATCH v3 13/13] KVM: selftests: Add a KVM_IRQFD test to verify uniqueness requirements

[Sairaj Kodilkar <sarunkod@xxxxxxx>]

On 5/23/2025 8:03 PM, Sean Christopherson wrote:
> On Fri, May 23, 2025, Sairaj Kodilkar wrote:
> > On 5/23/2025 5:22 AM, Sean Christopherson wrote:
> >
> > > +
> > > +int main(int argc, char *argv[])
> > > +{
> > > +       pthread_t racing_thread;
> > > +       int r, i;
> > > +
> > > +       /* Create "full" VMs, as KVM_IRQFD requires an in-kernel IRQ chip. */
> > > +       vm1 = vm_create(1);
> > > +       vm2 = vm_create(1);
> > > +
> > > +       WRITE_ONCE(__eventfd, kvm_new_eventfd());
> > > +
> > > +       kvm_irqfd(vm1, 10, __eventfd, 0);
> > > +
> > > +       r = __kvm_irqfd(vm1, 11, __eventfd, 0);
> > > +       TEST_ASSERT(r && errno == EBUSY,
> > > +                   "Wanted EBUSY, r = %d, errno = %d", r, errno);
> > > +
> > > +       r = __kvm_irqfd(vm2, 12, __eventfd, 0);
> > > +       TEST_ASSERT(r && errno == EBUSY,
> > > +                   "Wanted EBUSY, r = %d, errno = %d", r, errno);
> > > +
> > > +       kvm_irqfd(vm1, 11, READ_ONCE(__eventfd), KVM_IRQFD_FLAG_DEASSIGN);
> > > +       kvm_irqfd(vm1, 12, READ_ONCE(__eventfd), KVM_IRQFD_FLAG_DEASSIGN);
> > > +       kvm_irqfd(vm1, 13, READ_ONCE(__eventfd), KVM_IRQFD_FLAG_DEASSIGN);
> > > +       kvm_irqfd(vm1, 14, READ_ONCE(__eventfd), KVM_IRQFD_FLAG_DEASSIGN);
> > Hi Sean,
> > I dont see any allocation for the GSI 13 and 14..
> > Is there any reason for the deassigning these two GSIs ?
> Yes, KVM's rather bizarre ABI is that DEASSIGN is allowed even if the VM doesn't
> have a corresponding assigned irqfd.  The reason I added these early DEASSIGN
> calls is so that there will be an easier-to-debug failure if KVM's behavior
> changes (the racing threads part of the test abuses KVM's ABI).  I didn't add a
> comment because the helpers already have comments, but looking at this again, I
> agree that main() needs a better comment.
Makes sense, thanks for the explanation.

Thanks
Sairaj Kodilkar