Re: [RFC v1 00/22] Large rx buffer support for zcrx

[Mina Almasry <almasrymina@google.com>]

On Tue, Jul 29, 2025 at 9:41 AM Stanislav Fomichev <stfomichev@gmail.com> wrote:
>
> On 07/28, Mina Almasry wrote:
> > On Mon, Jul 28, 2025 at 3:06 PM Stanislav Fomichev <stfomichev@gmail.com> wrote:
> > >
> > > On 07/28, Pavel Begunkov wrote:
> > > > On 7/28/25 21:21, Stanislav Fomichev wrote:
> > > > > On 07/28, Pavel Begunkov wrote:
> > > > > > On 7/28/25 18:13, Stanislav Fomichev wrote:
> > > > ...>>> Supporting big buffers is the right direction, but I have the same
> > > > > > > feedback:
> > > > > >
> > > > > > Let me actually check the feedback for the queue config RFC...
> > > > > >
> > > > > > it would be nice to fit a cohesive story for the devmem as well.
> > > > > >
> > > > > > Only the last patch is zcrx specific, the rest is agnostic,
> > > > > > devmem can absolutely reuse that. I don't think there are any
> > > > > > issues wiring up devmem?
> > > > >
> > > > > Right, but the patch number 2 exposes per-queue rx-buf-len which
> > > > > I'm not sure is the right fit for devmem, see below. If all you
> > > >
> > > > I guess you're talking about uapi setting it, because as an
> > > > internal per queue parameter IMHO it does make sense for devmem.
> > > >
> > > > > care is exposing it via io_uring, maybe don't expose it from netlink for
> > > >
> > > > Sure, I can remove the set operation.
> > > >
> > > > > now? Although I'm not sure I understand why you're also passing
> > > > > this per-queue value via io_uring. Can you not inherit it from the
> > > > > queue config?
> > > >
> > > > It's not a great option. It complicates user space with netlink.
> > > > And there are convenience configuration features in the future
> > > > that requires io_uring to parse memory first. E.g. instead of
> > > > user specifying a particular size, it can say "choose the largest
> > > > length under 32K that the backing memory allows".
> > >
> > > Don't you already need a bunch of netlink to setup rss and flow
> > > steering? And if we end up adding queue api, you'll have to call that
> > > one over netlink also.
> > >
> >
> > I'm thinking one thing that could work is extending bind-rx with an
> > optional rx-buf-len arg, which in the code translates into devmem
> > using the new net_mp_open_rxq variant which not only restarts the
> > queue but also sets the size. From there the implementation should be
> > fairly straightforward in devmem. devmem currently rejects any pp for
> > which pp.order != 0. It would need to start accepting that and
> > forwarding the order to the gen_pool doing the allocations, etc.
>
> Right, that's the logical alternative, to put that rx-buf-len on the
> binding to control the size of the niovs. But then what do we do with
> the queue's rx-buf-len? bnxt patch in the series does
> page_pool_dev_alloc_frag(..., bp->rx_page_size). bp->rx_page_size comes
> from netlink. Does it need to be inherited from the pp in the devmem
> case somehow?

I need to review the series closely, but the only thing that makes
sense to me off the bat is that the rx-buf-len option sets the
rx-buf-len of the queue as if you called the queue-set API in a
separate call (and the unbind would reset the value to default).

-- 
Thanks,
Mina