Re: [PATCH rdma-next 2/2] RDMA/umem: block plain userspace memory registration under CoCo bounce

[Jason Gunthorpe <jgg@ziepe.ca>]

On Tue, May 05, 2026 at 09:20:01AM -0400, Jacob Moroni wrote:
> Hi,
> 
> Out of curiosity, it seems like we set DMA_ATTR_REQUIRE_COHERENT, so
> would that have caused these registrations to fail anyway since it would
> be trying to use swiotlb if running in a CVM?

It is supposed to, at least that is the intention. I think that
new attribute overtook Jiri's patch here?

> I was hoping that the new cc_shared heap could be used without
> modifying the kernel driver by replacing the normal allocations in the provider
> with a dmabuf allocation+mmap and just passing the resulting pointer to reg_mr,
> but that won't work because it's a PFN mapping.

> The driver could be modified to accept the actual dmabuf instead for the QP/CQ
> rings, but I just wanted to see if that matches your vision here or if
> you had something
> else in mind. 

Jiri has been looking at both options, but kernel side irdma must be
upgraded to accept a dmabuf for every kind of userspace memory.

This is why we have been trying to centralize more of the umem logic
because every driver should be upgraded to accept dmabuf for
everything...

> Another idea was to just allocate them in the kernel using the DMA
> allocator and map them into userspace but it would be a larger change.

This isn't the pattern we are using in rdma..

Jason