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

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Date:   Thu, 16 Nov 2023 09:42:23 +0800
From:   "Liu, Jing2" <jing2.liu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
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

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