On Wed, Jun 23, 2021 at 9:50 PM Jason Gunthorpe [off-list ref] wrote:

On Wed, Jun 23, 2021 at 09:43:04PM +0300, Oded Gabbay wrote:

quoted

Can you please explain why it is so important to (allow) access them
through the CPU ?

It is not so much important, as it reflects significant design choices
that are already tightly baked into alot of our stacks.

A SGL is CPU accessible by design - that is baked into this thing and
places all over the place assume it. Even in RDMA we have
RXE/SWI/HFI1/qib that might want to use the CPU side (grep for sg_page
to see)

So, the thing at the top of the stack - in this case the gaudi driver
- simply can't assume what the rest of the stack is going to do and
omit the CPU side. It breaks everything.

Logan's patch series is the most fully developed way out of this
predicament so far.

I understand the argument and I agree that for the generic case, the
top of the stack can't assume anything.
Having said that, in this case the SGL is encapsulated inside a dma-buf object.

Maybe its a stupid/over-simplified suggestion, but can't we add a
property to the dma-buf object,
that will be set by the exporter, which will "tell" the importer it
can't use any CPU fallback ? Only "real" p2p ?
Won't that solve the problem by eliminating the unsupported access methods ?

Oded

quoted

The whole purpose is that the other device accesses my device,
bypassing the CPU.

Sure, but you don't know that will happen, or if it is even possible
in any given system configuration. The purpose is to allow for that
optimization when possible, not exclude CPU based approaches.

Jason