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

[Sairaj Kodilkar]

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