Re: [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files

[Bas Nieuwenhuizen]

On Wed, Feb 26, 2020 at 4:29 PM Jason Ekstrand <jason@xxxxxxxxxxxxxx> wrote:
>
> On Wed, Feb 26, 2020 at 4:05 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
> >
> > On Wed, Feb 26, 2020 at 10:16:05AM +0100, Christian KÃnig wrote:
> > > Hi Jason,
> > >
> > > Am 26.02.20 um 00:58 schrieb Jason Ekstrand:
> > > > Explicit synchronization is the future.  At least, that seems to be what
> > > > most userspace APIs are agreeing on at this point.  However, most of our
> > > > Linux APIs (both userspace and kernel UAPI) are currently built around
> > > > implicit synchronization with dma-buf.  While work is ongoing to change
> > > > many of the userspace APIs and protocols to an explicit synchronization
> > > > model, switching over piecemeal is difficult due to the number of
> > > > potential components involved.  On the kernel side, many drivers use
> > > > dma-buf including GPU (3D/compute), display, v4l, and others.  In
> > > > userspace, we have X11, several Wayland compositors, 3D drivers, compute
> > > > drivers (OpenCL etc.), media encode/decode, and the list goes on.
> > > >
> > > > This patch provides a path forward by allowing userspace to manually
> > > > manage the fences attached to a dma-buf.  Alternatively, one can think
> > > > of this as making dma-buf's implicit synchronization simply a carrier
> > > > for an explicit fence.  This is accomplished by adding two IOCTLs to
> > > > dma-buf for importing and exporting a sync file to/from the dma-buf.
> > > > This way a userspace component which is uses explicit synchronization,
> > > > such as a Vulkan driver, can manually set the write fence on a buffer
> > > > before handing it off to an implicitly synchronized component such as a
> > > > Wayland compositor or video encoder.  In this way, each of the different
> > > > components can be upgraded to an explicit synchronization model one at a
> > > > time as long as the userspace pieces connecting them are aware of it and
> > > > import/export fences at the right times.
> > > >
> > > > There is a potential race condition with this API if userspace is not
> > > > careful.  A typical use case for implicit synchronization is to wait for
> > > > the dma-buf to be ready, use it, and then signal it for some other
> > > > component.  Because a sync_file cannot be created until it is guaranteed
> > > > to complete in finite time, userspace can only signal the dma-buf after
> > > > it has already submitted the work which uses it to the kernel and has
> > > > received a sync_file back.  There is no way to atomically submit a
> > > > wait-use-signal operation.  This is not, however, really a problem with
> > > > this API so much as it is a problem with explicit synchronization
> > > > itself.  The way this is typically handled is to have very explicit
> > > > ownership transfer points in the API or protocol which ensure that only
> > > > one component is using it at any given time.  Both X11 (via the PRESENT
> > > > extension) and Wayland provide such ownership transfer points via
> > > > explicit present and idle messages.
> > > >
> > > > The decision was intentionally made in this patch to make the import and
> > > > export operations IOCTLs on the dma-buf itself rather than as a DRM
> > > > IOCTL.  This makes it the import/export operation universal across all
> > > > components which use dma-buf including GPU, display, v4l, and others.
> > > > It also means that a userspace component can do the import/export
> > > > without access to the DRM fd which may be tricky to get in cases where
> > > > the client communicates with DRM via a userspace API such as OpenGL or
> > > > Vulkan.  At a future date we may choose to add direct import/export APIs
> > > > to components such as drm_syncobj to avoid allocating a file descriptor
> > > > and going through two ioctls.  However, that seems to be something of a
> > > > micro-optimization as import/export operations are likely to happen at a
> > > > rate of a few per frame of rendered or decoded video.
> > > >
> > > > Signed-off-by: Jason Ekstrand <jason@xxxxxxxxxxxxxx>
> > > > ---
> > > >
> > > > This is marked as an RFC because I intend it to start a discussion about
> > > > how to solve a problem.  The current patch compiles but that's it for now.
> > > > I'll be writing IGT tests and Vulkan driver patches which exercise it over
> > > > the next couple of days.  In the mean time, feel free to tell me why you
> > > > think this is a great and/or terrible idea. :-)
> > >
> > > For the exporting part I think it is an absolutely great idea because it
> > > simplifies compatibility with explicit sync quite a bit.
>
> Unfortunately, it only helps half of explicit sync and not the half
> that's hard to deal with from Vulkan. :-/
>
> > > But for the importing part it is a clear NAK at the moment. See we can't
> > > allow userspace to mess with DMA-buf fences in that way because it rips open
> > > a security hole you can push an elephant through.
>
> Oh, sure, I'm 100% sure I did that part wrong.  Why else would I send
> the patch but to have someone who actually knows what they're doing
> tell me how to do it correctly? :-P
>
> > > Just imagine that you access some DMA-buf with a shader and that operation
> > > is presented as a fence on the DMA-bufs reservation object. And now you can
> > > go ahead and replace that fence and free up the memory.
> > >
> > > Tricking the Linux kernel into allocating page tables in that freed memory
> > > is trivial and that's basically it you can overwrite page tables with your
> > > shader and gain access to all of system memory :)
> > >
> > > What we could do is to always make sure that the added fences will complete
> > > later than the already existing ones, but that is also rather tricky to get
> > > right. I wouldn't do that if we don't have a rather big use case for this.
>
> Right.  I thought about that but I'm still learning how dma_resv
> works.  It'd be easy enough to make a fence array that contains both
> the old fence and the new fence and replace the old fence with that.
> What I don't know is the proper way to replace the exclusive fence
> safely.  Some sort of atomic_cpxchg loop, perhaps?  I presume there's
> some way of doing it properly because DRM drivers are doing it all the
> time.

I think for an exclusive fence you may need to create a fence array
that includes the existing exclusive and shared fences in the dma_resv
combined with the added fence.

However, I'm not sure what the best way is to do garbage collection on
that so that we don't get an impossibly list of fence arrays. (Note
the dma_resv has a lock that needs to be taken before adding an
exclusive fence, might be useful). Some code that does a thing like
this is __dma_resv_make_exclusive in
drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c

The other piece of the puzzle is that on the submit path this would
need something to ignore implicit fences. And there semantically the
question comes up whether it is safe for a driver to ignore exclusive
fences from another driver. (and then we have amdgpu which has its own
rules on exclusiveness of its shared fences based on the context. e.g.
the current option to ignore implicit fences for a buffer still syncs
on exclusive fences on the buffer).









>
> > I think the main use-case for adding a fence is adding a write fence for
> > vk winsys buffers, which run without any sync at all. So essentially what
> > we'd do is promote one of the read fences which are already attached to be
> > the write fence.
>
> Correct.  We're effectively doing an import in ANV today but we're
> doing it with a dummy execbuf which claims to write the BO and has a
> batch that's just MI_BATCH_BUFFER_END.
>
> > But yeah making sure we don't break any of the dma_resv guarantees about
> > how these fences works is going to be somewhat tricky. Probably can reuse
> > a big chunk of the fence container work we've done for syncobj timelines,
> > since they have some of the same issues of having to chain fences to not
> > break the world.
>
> Happy to not break the world.  I just don't know how yet. :-)
>
> --Jason