Re: [RFC PATCH v3 07/20] io_uring: add interface queue

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

David Wei wrote:
> From: David Wei <davidhwei@meta.com>
> 
> This patch introduces a new object in io_uring called an interface queue
> (ifq) which contains:
> 
> * A pool region allocated by userspace and registered w/ io_uring where
>   Rx data is written to.
> * A net device and one specific Rx queue in it that will be configured
>   for ZC Rx.
> * A pair of shared ringbuffers w/ userspace, dubbed registered buf
>   (rbuf) rings. Each entry contains a pool region id and an offset + len
>   within that region. The kernel writes entries into the completion ring
>   to tell userspace where RX data is relative to the start of a region.
>   Userspace writes entries into the refill ring to tell the kernel when
>   it is done with the data.
> 
> For now, each io_uring instance has a single ifq, and each ifq has a
> single pool region associated with one Rx queue.
> 
> Add a new opcode to io_uring_register that sets up an ifq. Size and
> offsets of shared ringbuffers are returned to userspace for it to mmap.
> The implementation will be added in a later patch.
> 
> Signed-off-by: David Wei <dw@davidwei.uk>

This is quite similar to AF_XDP, of course. Is it at all possible to
reuse all or some of that? If not, why not?

As a side effect, unification would also show a path of moving AF_XDP
from its custom allocator to the page_pool infra.

Related: what is the story wrt the process crashing while user memory
is posted to the NIC or present in the kernel stack.

SO_DEVMEM already demonstrates zerocopy into user buffers using usdma.
To a certain extent that and asyncronous I/O with iouring are two
independent goals. SO_DEVMEM imposes limitations on the stack because
it might hold opaque device mem. That is too strong for this case.

But for this iouring provider, is there anything ioring specific about
it beyond being user memory? If not, maybe just call it a umem
provider, and anticipate it being usable for AF_XDP in the future too?

Besides delivery up to the intended socket, packets may also end up
in other code paths, such as packet sockets or forwarding. All of
this is simpler with userspace backed buffers than with device mem.
But good to call out explicitly how this is handled. MSG_ZEROCOPY
makes a deep packet copy in unexpected code paths, for instance. To
avoid indefinite latency to buffer reclaim.