Re: [RFC PATCH v3 10/12] tcp: RX path for devmem TCP

[Edward Cree <ecree.xilinx@gmail.com>]

On 09/11/2023 02:39, Mina Almasry wrote:
> On Wed, Nov 8, 2023 at 7:36 AM Edward Cree <ecree.xilinx@gmail.com> wrote:
>>  If not then surely the way to return a memory area
>>  in an io_uring idiom is just to post a new read sqe ('RX descriptor')
>>  pointing into it, rather than explicitly returning it with setsockopt.
> 
> We're interested in using this with regular TCP sockets, not
> necessarily io_uring.
Fair.  I just wanted to push against the suggestion upthread that "oh,
 since io_uring supports setsockopt() we can just ignore it and it'll
 all magically work later" (paraphrased).
If you can keep the "allocate buffers out of a devmem region" and "post
 RX descriptors built on those buffers" APIs separate (inside the
 kernel; obviously both triggered by a single call to the setsockopt()
 uAPI) that'll likely make things simpler for the io_uring interface I
 describe, which will only want the latter.

-ed

PS: Here's a crazy idea that I haven't thought through at all: what if
 you allow device memory to be mmap()ed into process address space
 (obviously with none of r/w/x because it's unreachable), so that your
 various uAPIs can just operate on pointers (e.g. the setsockopt
 becomes the madvise it's named after; recvmsg just uses or populates
 the iovec rather than needing a cmsg).  Then if future devices have
 their memory CXL accessible that can potentially be enabled with no
 change to the uAPI (userland just starts being able to access the
 region without faulting).
And you can maybe add a semantic flag to recvmsg saying "if you don't
 use all the buffers in my iovec, keep hold of the rest of them for
 future incoming traffic, and if I post new buffers with my next
 recvmsg, add those to the tail of the RXQ rather than replacing the
 ones you've got".  That way you can still have the "userland
 directly fills the RX ring" behaviour even with TCP sockets.