Re: [PATCH v2 0/6] IOMMUFD: Deliver IO page faults to user space

[Liu, Jing2]

Hi Jason,

On 11/15/2023 9:58 PM, Jason Gunthorpe wrote:
> On Wed, Nov 15, 2023 at 01:17:06PM +0800, Liu, Jing2 wrote:
>
> > This is the right way to approach it,
> >
> >     I learned that there was discussion about using io_uring to get the
> >     page fault without
> >
> >     eventfd notification in [1], and I am new at io_uring and studying the
> >     man page of
> >
> >     liburing, but there're questions in my mind on how can QEMU get the
> >     coming page fault
> >
> >     with a good performance.
> >
> >     Since both QEMU and Kernel don't know when comes faults, after QEMU
> >     submits one
> >
> >     read task to io_uring, we want kernel pending until fault comes. While
> >     based on
> >
> >     hwpt_fault_fops_read() in [patch v2 4/6], it just returns 0 since
> >     there's now no fault,
> >
> >     thus this round of read completes to CQ but it's not what we want. So
> >     I'm wondering
> >
> >     how kernel pending on the read until fault comes. Does fops callback
> >     need special work to
> Implement a fops with poll support that triggers when a new event is
> pushed and everything will be fine.

Does userspace need also setup a POLL flag to let io_uring go into poll, 
or io_uring

always try to poll?

> There are many examples in the
> kernel. The ones in the mlx5 vfio driver spring to mind as a scheme I
> recently looked at.

Thank you very much for guiding the way. We will study the example to 
understand

more.

BRs,

Jing

> Jason