Re: New subsystem for acceleration devices

[Arnd Bergmann <arnd@arndb.de>]

On Tue, Aug 9, 2022 at 10:04 AM Christoph Hellwig <hch@infradead.org> wrote:
> On Tue, Aug 09, 2022 at 08:23:27AM +0200, Greg Kroah-Hartman wrote:
> > > This is different from the number of FDs pointing at the struct file.
> > > Userpsace can open a HW state and point a lot of FDs at it, that is
> > > userspace's problem. From a kernel view they all share one struct file
> > > and thus one HW state.
> >
> > Yes, that's fine, if that is what is happening here, I have no
> > objection.
>
> It would be great if we could actually life that into a common
> layer (chardev or vfs) given just how common this, and drivers tend
> to get it wrong, do it suboptimal so often.

Totally agreed.

I think for devices with hardware MMU contexts you actually want to
bind the context to a 'mm_struct', and then ensure that the context
is only ever used from a process that shares this mm_struct,
regardless of who else has access to the same file or inode.

We did this in a somewhat hacky way in spufs a long time ago, and
I would expect this to be solved more cleanly in drivers/gpu/drm, but
I don't know where to look for that code.

        Arnd