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

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Date:   Mon, 20 Nov 2023 20:14:52 -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 Thu, Nov 16, 2023 at 09:42:23AM +0800, Liu, Jing2 wrote:
> 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?

io_uring can trigger poll and use other approaches, it is flexible the
driver can scale this in different ways.

Jason