Re: [PATCH rdma-core 3/5] pyverbs: Add dma-buf based MR support

Daniel Vetter
2020-11-24T15:17:05Z

On Mon, Nov 23, 2020 at 02:05:04PM -0400, Jason Gunthorpe wrote:
> On Mon, Nov 23, 2020 at 09:53:02AM -0800, Jianxin Xiong wrote:
>
> > +cdef class DmaBuf:
> > + def __init__(self, size, unit=0):
> > + """
> > + Allocate DmaBuf object from a GPU device. This is done through the
> > + DRI device interface (/dev/dri/card*). Usually this requires the

Please use /dev/dri/renderD* instead. That's the interface meant for
unpriviledged rendering access. card* is the legacy interface with
backwards compat galore, don't use.

Specifically if you do this on a gpu which also has display (maybe some
testing on a local developer machine, no idea ...) then you mess with
compositors and stuff.

Also wherever you copied this from, please also educate those teams that
using /dev/dri/card* for rendering stuff is a Bad Idea (tm)

> > + effective user id being root or being a member of the 'video' group.
> > + :param size: The size (in number of bytes) of the buffer.
> > + :param unit: The unit number of the GPU to allocate the buffer from.
> > + :return: The newly created DmaBuf object on success.
> > + """
> > + self.dmabuf_mrs = weakref.WeakSet()
> > + self.dri_fd = open('/dev/dri/card'+str(unit), O_RDWR)
> > +
> > + args = bytearray(32)
> > + pack_into('=iiiiiiq', args, 0, 1, size, 8, 0, 0, 0, 0)
> > + ioctl(self.dri_fd, DRM_IOCTL_MODE_CREATE_DUMB, args)
> > + a, b, c, d, self.handle, e, self.size = unpack('=iiiiiiq', args)

Yeah no, don't allocate render buffers with create_dumb. Every time this
comes up I'm wondering whether we should just completely disable dma-buf
operations on these. Dumb buffers are explicitly only for software
rendering for display purposes when the gpu userspace stack isn't fully
running yet, aka boot splash.

And yes I know there's endless amounts of abuse of that stuff floating
around, especially on arm-soc/android systems.

> > +
> > + args = bytearray(12)
> > + pack_into('=iii', args, 0, self.handle, O_RDWR, 0)
> > + ioctl(self.dri_fd, DRM_IOCTL_PRIME_HANDLE_TO_FD, args)
> > + a, b, self.fd = unpack('=iii', args)
> > +
> > + args = bytearray(16)
> > + pack_into('=iiq', args, 0, self.handle, 0, 0)
> > + ioctl(self.dri_fd, DRM_IOCTL_MODE_MAP_DUMB, args);
> > + a, b, self.map_offset = unpack('=iiq', args);
>
> Wow, OK
>
> Is it worth using ctypes here instead? Can you at least add a comment
> before each pack specifying the 'struct XXX' this is following?
>
> Does this work with normal Intel GPUs, like in a Laptop? AMD too?
>
> Christian, I would be very happy to hear from you that this entire
> work is good for AMD as well

I think the smallest generic interface for allocating gpu buffers which
are more useful than the stuff you get from CREATE_DUMB is gbm. That's
used by compositors to get bare metal opengl going on linux. Ofc Android
has gralloc for the same purpose, and cros has minigbm (which isn't the
same as gbm at all). So not so cool.

The other generic option is using vulkan, which works directly on bare
metal (without a compositor or anything running), and is cross vendor. So
cool, except not used for compute, which is generally the thing you want
if you have an rdma card.

Both gbm-egl/opengl and vulkan have extensions to hand you a dma-buf back,
properly.

Compute is the worst, because opencl is widely considered a mistake (maybe
opencl 3 is better, but nvidia is stuck on 1.2). The actually used stuff is
cuda (nvidia-only), rocm (amd-only) and now with intel also playing we
have xe (intel-only).

It's pretty glorious :-/

Also I think we discussed this already, but for actual p2p the intel
patches aren't in upstream yet. We have some internally, but with very
broken locking (in the process of getting fixed up, but it's taking time).

Cheers, Daniel

> Edward should look through this, but I'm glad to see something like
> this
>
> Thanks,
> Jason
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--
Daniel Vetter
Software Engineer, Intel Corporation