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

[Mina Almasry]

On Mon, Nov 6, 2023 at 4:03 PM Willem de Bruijn
<willemdebruijn.kernel@xxxxxxxxx> wrote:
>
> On Mon, Nov 6, 2023 at 3:55 PM David Ahern <dsahern@xxxxxxxxxx> wrote:
> >
> > On 11/6/23 4:32 PM, Stanislav Fomichev wrote:
> > >> The concise notification API returns tokens as a range for
> > >> compression, encoding as two 32-bit unsigned integers start + length.
> > >> It allows for even further batching by returning multiple such ranges
> > >> in a single call.
> > >
> > > Tangential: should tokens be u64? Otherwise we can't have more than
> > > 4gb unacknowledged. Or that's a reasonable constraint?
> > >
> >
> > Was thinking the same and with bits reserved for a dmabuf id to allow
> > multiple dmabufs in a single rx queue (future extension, but build the
> > capability in now). e.g., something like a 37b offset (128GB dmabuf
> > size), 19b length (large GRO), 8b dmabuf id (lots of dmabufs to a queue).
>
> Agreed. Converting to 64b now sounds like a good forward looking revision.

The concept of IDing a dma-buf came up in a couple of different
contexts. First, in the context of us giving the dma-buf ID to the
user on recvmsg() to tell the user the data is in this specific
dma-buf. The second context is here, to bind dma-bufs with multiple
user-visible IDs to an rx queue.

My issue here is that I don't see anything in the struct dma_buf that
can practically serve as an ID:

https://elixir.bootlin.com/linux/v6.6-rc7/source/include/linux/dma-buf.h#L302

Actually, from the userspace, only the name of the dma-buf seems
queryable. That's only unique if the user sets it as such. The dmabuf
FD can't serve as an ID. For our use case we need to support 1 process
doing the dma-buf bind via netlink, sharing the dma-buf FD to another
process, and that process receives the data.  In this case the FDs
shown by the 2 processes may be different. Converting to 64b is a
trivial change I can make now, but I'm not sure how to ID these
dma-bufs. Suggestions welcome. I'm not sure the dma-buf guys will
allow adding a new ID + APIs to query said dma-buf ID.

--
Thanks,
Mina