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

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Date:   Mon, 1 Nov 2021 09:50:13 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "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>,
        "Tian, Kevin" <kevin.tian@...el.com>,
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Subject: Re: [RFC 02/20] vfio: Add device class for /dev/vfio/devices

On Fri, Oct 29, 2021 at 09:47:27AM +0000, Liu, Yi L wrote:
> Hi Jason,
> 
> > From: Jason Gunthorpe <jgg@...dia.com>
> > Sent: Monday, October 25, 2021 8:53 PM
> > 
> > On Mon, Oct 25, 2021 at 06:28:09AM +0000, Liu, Yi L wrote:
> > >    thanks for the guiding. will also refer to your vfio_group_cdev series.
> > >
> > >    Need to double confirm here. Not quite following on the kfree. Is
> > >    this kfree to free the vfio_device structure? But now the
> > >    vfio_device pointer is provided by callers (e.g. vfio-pci). Do
> > >    you want to let vfio core allocate the vfio_device struct and
> > >    return the pointer to callers?
> > 
> > There are several common patterns for this problem, two that would be
> > suitable:
> > 
> > - Require each driver to provide a release op inside vfio_device_ops
> >   that does the kfree. Have the core provide a struct device release
> >   op that calls this one. Keep the kalloc/kfree in the drivers
> 
> this way sees to suit the existing vfio registration manner listed
> below. right? 

Not really, most drivers are just doing kfree. The need for release
comes if the drivers are doing more stuff.

> But device drivers needs to do the kfree in the
> newly added release op instead of doing it on their own (e.g.
> doing kfree in remove).

Yes

> > struct ib_device *_ib_alloc_device(size_t size);
> > #define ib_alloc_device(drv_struct, member)                                    \
> >         container_of(_ib_alloc_device(sizeof(struct drv_struct) +              \
> >                                       BUILD_BUG_ON_ZERO(offsetof(              \
> >                                               struct drv_struct, member))),    \
> >                      struct drv_struct, member)
> > 
> 
> thanks for the example. If this way, still requires driver to provide
> a release op inside vfio_device_ops. right?

No, it would optional. It would contain the stuff the driver is doing
before kfree()

For instance mdev looks like the only driver that cares:

	vfio_uninit_group_dev(&mdev_state->vdev);
	kfree(mdev_state->pages);
	kfree(mdev_state->vconfig);
	kfree(mdev_state);

pages/vconfig would logically be in a release function

On the other hand ccw needs to rcu free the vfio_device, so that would
have to be global overhead with this api design.

Jason