Hey Christoph, Haggai,

quoted

Allocate a CMB region to user-space as a DMA-BUF object.

NACK.  Absolutely no way we're going to export a low-level device
specific feature to userspace like that.  If you want to DMA to
the CMB do it from the kernel, and for that we'll need a proper
block device level API.

I think that in this patch, Haggai was aiming to demonstrate the use
of dma_buf for peer2peer transactions with an RDMA device. I agree
that the CMB example can/should be exposed in the block device level,
but I also think we'd like to generalize the way we expose a bar space
of any peer device to another peer device.

How would you suggest we do that? I think Haggai's proposal is pretty
solid (considering the fact that the vast majority of RDMA applications
live in user-space).