Re: [Intel-gfx] Allow mdev drivers to directly create the vfio_device (v3)

From: Alex Williamson <alex.williamson@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: kvm@vger.kernel.org, linux-doc@vger.kernel.org,
	David Airlie <airlied@linux.ie>,
	dri-devel@lists.freedesktop.org,
	Kirti Wankhede <kwankhede@nvidia.com>,
	linux-s390@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Halil Pasic <pasic@linux.ibm.com>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Jason Gunthorpe <jgg@nvidia.com>,
	intel-gfx@lists.freedesktop.org,
	Jason Herne <jjherne@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Heiko Carstens <hca@linux.ibm.com>,
	Tony Krowiak <akrowiak@linux.ibm.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Cornelia Huck <cohuck@redhat.com>
Subject: Re: [Intel-gfx] Allow mdev drivers to directly create the vfio_device (v3)
Date: Tue, 15 Jun 2021 13:35:49 -0600	[thread overview]
Message-ID: <20210615133549.362e5a9e.alex.williamson@redhat.com> (raw)
In-Reply-To: <20210615133519.754763-1-hch@lst.de>

On Tue, 15 Jun 2021 15:35:09 +0200
Christoph Hellwig <hch@lst.de> wrote:

> This is my alternative take on this series from Jason:
> 
> https://lore.kernel.org/dri-devel/87czsszi9i.fsf@redhat.com/T/
> 
> The mdev/vfio parts are exactly the same, but this solves the driver core
> changes for the direct probing without the in/out flag that Greg hated,
> which cause a little more work, but probably make the result better.
> 
> Original decription from Jason below:
> 
> The mdev bus's core part for managing the lifecycle of devices is mostly
> as one would expect for a driver core bus subsystem.
> 
> However instead of having a normal 'struct device_driver' and binding the
> actual mdev drivers through the standard driver core mechanisms it open
> codes this with the struct mdev_parent_ops and provides a single driver
> that shims between the VFIO core's struct vfio_device and the actual
> device driver.
> 
> Instead, allow mdev drivers implement an actual struct mdev_driver and
> directly call vfio_register_group_dev() in the probe() function for the
> mdev. Arrange to bind the created mdev_device to the mdev_driver that is
> provided by the end driver.
> 
> The actual execution flow doesn't change much, eg what was
> parent_ops->create is now device_driver->probe and it is called at almost
> the exact same time - except under the normal control of the driver core.
> 
> Ultimately converting all the drivers unlocks a fair number of additional
> VFIO simplifications and cleanups.

Looks like we need an update to
Documentation/driver-api/vfio-mediated-device.rst to go along with
this.

Also, if we're preserving compatibility with the "legacy"
mdev_parent_ops callbacks without deprecating them, does it really make
sense to convert every one of the sample drivers to this new direct
registration?  Thanks,

Alex

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