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

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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




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