Re: [PATCH v5 08/14] iommufd/viommu: Add iommufd_viommu_report_event helper

[Jason Gunthorpe <jgg@nvidia.com>]

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:
> 
> 1. 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.

> 2. 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