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

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Message-ID: <ZVTObiybC1MYlSam@ziepe.ca>
Date:   Wed, 15 Nov 2023 09:58:06 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "Liu, Jing2" <jing2.liu@...ux.intel.com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Nicolin Chen <nicolinc@...dia.com>,
        Yi Liu <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        iommu@...ts.linux.dev, linux-kselftest@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/6] IOMMUFD: Deliver IO page faults to user space

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

Jason