Re: [PATCH 10/13] vfio: Make vfio_device_open() exclusive between group path and device cdev path

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Feb 08, 2023 at 04:23:16AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Tuesday, February 7, 2023 9:27 PM
> > > >
> > > > No, I mean why can't vfio just call iommufd exactly once regardless of
> > > > what mode it is running in?
> > >
> > > This seems to be moving the DMA owner claim out of
> > iommufd_device_bind().
> > > Is it? Then either group and cdev can claim DMA owner with their own
> > DMA
> > > marker.
> > 
> > No, it has nothing to do with DMA ownership
> > 
> > Just keep a flag in vfio saying it is in group mode or device mode and
> > act accordingly.
> 
> It cannot be a simple flag. needs to be a refcnt since multiple devices 
> in the group might be opened via cdev so the device mode should be
> cleared only when the last device via cdev is closed.
> 
> Yi actually did implement such a flavor before, kind of introducing
> a vfio_group->cdev_opened_cnt field.
> 
> Then cdev bind_iommufd checks whether vfio_group->opened_file
> has been set in the group open path. If not then increment
> vfio_group->cdev_opened_cnt.
> 
> cdev close decrements vfio_group->cdev_opened_cnt.
> 
> group open checks whether vfio_group->cdev_opened_cnt has been
> set. If not go to set vfio_group->opened_file.
> 
> In this case only one path can claim DMA ownership.
> 
> Is above what you expect?

It seems appropriate

You could also sweep the device list to see how the indivudal devices are
open to decice what to do.

> > The iommufd DMA owner check is *only* to be used for protecting
> > against two unrelated drivers trying to claim the same device.
> > 
> 
> this is just one implementation choice. I don't see why it cannot be
> extended to allow one driver to protect against two internal paths.
> Just simply allow the driver to assign an owner instead of assuming
> iommufd_ctx.

It is really not what it is for, and the owner thing is so ugly I
don't like the pattern. 

Jason