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

[David Ahern <dsahern@kernel.org>]

On 11/7/23 4:55 PM, Mina Almasry wrote:
> On Mon, Nov 6, 2023 at 4:03 PM Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
>>
>> On Mon, Nov 6, 2023 at 3:55 PM David Ahern <dsahern@kernel.org> 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.
> 

The API can be unique to this usage: e.g., add a dmabuf id to the
netlink API. Userspace manages the ids (tells the kernel what value to
use with an instance), the kernel validates no 2 dmabufs have the same
id and then returns the value here.