Re: [PATCH v3 4/8] iommufd: Add iommufd fault object

[Baolu Lu]

On 3/25/24 11:26 AM, Baolu Lu wrote:
> On 3/23/24 1:22 AM, Jason Gunthorpe wrote:
> > On Wed, Mar 20, 2024 at 04:18:05PM +0000, Shameerali Kolothum Thodi 
> > wrote:
> > > What I have noticed is that,
> > > -read interface works fine and I can receive struct 
> > > tiommu_hwpt_pgfault data.
> > > -But once Guest handles the page faults and returns the page response,
> > >   the write to fault fd never reaches the kernel. The sequence is 
> > > like below,
> > >    sqe = io_uring_get_sqe(ring);
> > >    io_uring_prep_write(sqe, hwpt->fault_fd, resp, sizeof(*resp), 0);
> > >    io_uring_sqe_set_data(sqe, resp);
> > >    io_uring_submit(ring);
> > >    ret = io_uring_wait_cqe(ring, &cqe);
> > >    ....
> > > Please find the function here[2]
> > >
> > > The above cqe wait never returns and hardware times out without 
> > > receiving
> > > page response. My understanding of io_uring default op is that it 
> > > tries to
> > > issue an sqe as non-blocking first. But it looks like the above write 
> > > sequence
> > > ends up in kernel poll_wait() as well.Not sure how we can avoid that for
> > > write.
> > Ah, right, it is because poll can't be choosy about read/write, it has
> > to work equally for both directions. iommufd_fault_fops_poll() never
> > returns EPOLLOUT
> >
> > It should just always return EPOLLOUT because we don't have any queue
> > to manage.
>
> Are you suggesting the poll file operation to be like below?
>
> static __poll_t iommufd_fault_fops_poll(struct file *filep,
>                                          struct poll_table_struct *wait)
> {
>          struct iommufd_fault *fault = filep->private_data;
>          __poll_t pollflags = EPOLLOUT;
>
>          poll_wait(filep, &fault->wait_queue, wait);
>          mutex_lock(&fault->mutex);
>          if (!list_empty(&fault->deliver))
>                  pollflags = EPOLLIN | EPOLLRDNORM;

If I understood it correctly, here it should be,

		pollflags |= EPOLLIN | EPOLLRDNORM;

>          mutex_unlock(&fault->mutex);
>
>          return pollflags;
> }

Best regards,
baolu