On Sun, Jan 12, 2025 at 09:37:41PM -0800, Nicolin Chen wrote:
I supposed we will need a way to indicate lost events to userspace on top of this?
Perhaps another u32 flag in the arm_smmuv3_vevent struct to report an overflow. That said, what userspace/VMM will need to do with it?
Trigger the above code in the VM somehow?
I found two ways of forwarding an overflow flag:
- Return -EOVERFLOW to read(). But it cannot return the read bytes
any more:
You could not return any bytes, it would have to be 0 bytes read, ie immediately return EOVERFLOW and do nothing else.
Returning EOVERFLOW from read would have to also clear the overflow indicator.
The other approach would be to add a sequence number to each event and let userspace detect the non-montonicity. It would require adding a header to the native ARM evt.
- Return EPOLLERR via pollfd.revents. But it cannot use POLLERR
for other errors any more:
@@ -420,2 +421,4 @@ static __poll_t iommufd_eventq_fops_poll(struct file *filep, poll_wait(filep, &eventq->wait_queue, wait);
if (test_bit(IOMMUFD_VEVENTQ_ERROR_OVERFLOW, veventq->errors))return EPOLLERR; mutex_lock(&eventq->mutex);
But then how do you clear the error? I've only seen POLLERR used for fatal conditions so there is no recovery, it is permanent.
Jason