On Mon, Jul 07, 2025 at 12:15:54PM +0100, Pavel Begunkov wrote:

...

to attach to / detach from a dma_buf, and then have an iter that specifies a dmabuf and offsets into. That way the code behind the file operations can forward the attachment to all the needed devices (including more/less while it remains attached to the file) and can pick the right dma address for each device.

By "iter that specifies a dmabuf" do you mean an opaque file-specific structure allocated inside the new fop?

I mean a reference the actual dma_buf (probably indirect through the file * for it, but listen to the dma_buf experts for that and not me).

Akin to what Keith proposed back then. That sounds good and has more potential for various optimisations. My concern would be growing struct iov_iter by an extra pointer:

struct iov_iter { union { struct iovec *iov; struct dma_seg *dmav; ... }; void *dma_token; };

But maybe that's fine. It's 40B -> 48B,

Alternatively we could the union point to a struct that has the dma buf pointer and a variable length array of dma_segs. Not sure if that would create a mess in the callers, though.

and it'll get back to 40 when / if xarray_start / ITER_XARRAY is removed.

Would it? At least for 64-bit architectures nr_segs is the same size.