linux-kernel - RE: [RFC 02/20] vfio: Add device class for /dev/vfio/devices

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Date:   Tue, 21 Sep 2021 23:56:06 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>, "Liu, Yi L" <yi.l.liu@...el.com>
CC:     "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "hch@....de" <hch@....de>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "parav@...lanox.com" <parav@...lanox.com>,
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        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
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Subject: RE: [RFC 02/20] vfio: Add device class for /dev/vfio/devices

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Tuesday, September 21, 2021 11:57 PM
> 
> On Sun, Sep 19, 2021 at 02:38:30PM +0800, Liu Yi L wrote:
> > This patch introduces a new interface (/dev/vfio/devices/$DEVICE) for
> > userspace to directly open a vfio device w/o relying on container/group
> > (/dev/vfio/$GROUP). Anything related to group is now hidden behind
> > iommufd (more specifically in iommu core by this RFC) in a device-centric
> > manner.
> >
> > In case a device is exposed in both legacy and new interfaces (see next
> > patch for how to decide it), this patch also ensures that when the device
> > is already opened via one interface then the other one must be blocked.
> >
> > Signed-off-by: Liu Yi L <yi.l.liu@...el.com>
> > ---
> >  drivers/vfio/vfio.c  | 228 +++++++++++++++++++++++++++++++++++++++----
> >  include/linux/vfio.h |   2 +
> >  2 files changed, 213 insertions(+), 17 deletions(-)
> 
> > +static int vfio_init_device_class(void)
> > +{
> > +	int ret;
> > +
> > +	mutex_init(&vfio.device_lock);
> > +	idr_init(&vfio.device_idr);
> > +
> > +	/* /dev/vfio/devices/$DEVICE */
> > +	vfio.device_class = class_create(THIS_MODULE, "vfio-device");
> > +	if (IS_ERR(vfio.device_class))
> > +		return PTR_ERR(vfio.device_class);
> > +
> > +	vfio.device_class->devnode = vfio_device_devnode;
> > +
> > +	ret = alloc_chrdev_region(&vfio.device_devt, 0, MINORMASK + 1,
> "vfio-device");
> > +	if (ret)
> > +		goto err_alloc_chrdev;
> > +
> > +	cdev_init(&vfio.device_cdev, &vfio_device_fops);
> > +	ret = cdev_add(&vfio.device_cdev, vfio.device_devt, MINORMASK +
> 1);
> > +	if (ret)
> > +		goto err_cdev_add;
> 
> Huh? This is not how cdevs are used. This patch needs rewriting.
> 
> The struct vfio_device should gain a 'struct device' and 'struct cdev'
> as non-pointer members
> 
> vfio register path should end up doing cdev_device_add() for each
> vfio_device
> 
> vfio_unregister path should do cdev_device_del()
> 
> No idr should be needed, an ida is used to allocate minor numbers
> 
> The struct device release function should trigger a kfree which
> requires some reworking of the callers
> 
> vfio_init_group_dev() should do a device_initialize()
> vfio_uninit_group_dev() should do a device_put()

All above are good suggestions!

> 
> The opened atomic is aweful. A newly created fd should start in a
> state where it has a disabled fops
> 
> The only thing the disabled fops can do is register the device to the
> iommu fd. When successfully registered the device gets the normal fops.
> 
> The registration steps should be done under a normal lock inside the
> vfio_device. If a vfio_device is already registered then further
> registration should fail.
> 
> Getting the device fd via the group fd triggers the same sequence as
> above.
> 

Above works if the group interface is also connected to iommufd, i.e.
making vfio type1 as a shim. In this case we can use the registration
status as the exclusive switch. But if we keep vfio type1 separate as
today, then a new atomic is still necessary. This all depends on how
we want to deal with vfio type1 and iommufd, and possibly what's
discussed here just adds another pound to the shim option...

Thanks
Kevin