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

[Sean Christopherson <seanjc@google.com>]

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.