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;
mutex_unlock(&fault->mutex);
return pollflags;
}
The diff is,
diff --git a/drivers/iommu/iommufd/fault.c b/drivers/iommu/iommufd/fault.c
index ede16702d433..a33f8aa92575 100644
--- a/drivers/iommu/iommufd/fault.c
+++ b/drivers/iommu/iommufd/fault.c
@@ -175,7 +175,7 @@ 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 = 0;
+ __poll_t pollflags = EPOLLOUT;
poll_wait(filep, &fault->wait_queue, wait);
mutex_lock(&fault->mutex);
I was originally thinking that poll file operation is specifically
designed for polling on read events associated with IOMMU faults.
Best regards,
baolu
