Re: [Linaro-mm-sig] [PATCH 04/18] dma-fence: prime lockdep annotations

Jason Gunthorpe
2020-06-18T17:23:48Z

On Thu, Jun 18, 2020 at 05:00:51PM +0200, Daniel Vetter wrote:
> On Wed, Jun 17, 2020 at 12:28:35PM -0300, Jason Gunthorpe wrote:
> > On Wed, Jun 17, 2020 at 08:48:50AM +0200, Daniel Vetter wrote:
> >
> > > Now my understanding for rdma is that if you don't have hw page fault
> > > support,
> >
> > The RDMA ODP feature is restartable HW page faulting just like nouveau
> > has. The classical MR feature doesn't have this. Only mlx5 HW supports
> > ODP today.
> >
> > > It's only gpus (I think) which are in this awkward in-between spot
> > > where dynamic memory management really is much wanted, but the hw
> > > kinda sucks. Aside, about 10+ years ago we had a similar problem with
> > > gpu hw, but for security: Many gpu didn't have any kinds of page
> > > tables to isolate different clients from each another. drivers/gpu
> > > fixed this by parsing&validating what userspace submitted to make sure
> > > it's only every accessing its own buffers. Most gpus have become
> > > reasonable nowadays and do have proper per-process pagetables (gpu
> > > process, not the pasid stuff), but even today there's still some of
> > > the old model left in some of the smallest SoC.
> >
> > But I still don't understand why a dma fence is needed inside the GPU
> > driver itself in the notifier.
> >
> > Surely the GPU driver can block and release the notifier directly from
> > its own command processing channel?
> >
> > Why does this fence and all it entails need to leak out across
> > drivers?
>
> So 10 years ago we had this world of every gpu driver is its own bucket,
> nothing leaks out to the world. But the world had a different idea how
> gpus where supposed to work, with stuff like:

Sure, I understand DMA fence, but why does a *notifier* need it?

The job of the notifier is to guarentee that the device it is
connected to is not doing DMA before it returns.

That just means you need to prove that device is done with the buffer.

As I've understood GPU that means you need to show that the commands
associated with the buffer have completed. This is all local stuff
within the driver, right? Why use fence (other than it already exists)

Jason