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

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Message-ID: <aIj5nuJJy1FVqbjC@mini-arch>
Date: Tue, 29 Jul 2025 09:41:02 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: Pavel Begunkov <asml.silence@...il.com>,
	Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
	io-uring@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>, andrew+netdev@...n.ch,
	horms@...nel.org, davem@...emloft.net, sdf@...ichev.me,
	dw@...idwei.uk, michael.chan@...adcom.com, dtatulea@...dia.com,
	ap420073@...il.com
Subject: Re: [RFC v1 00/22] Large rx buffer support for zcrx

On 07/28, Mina Almasry wrote:
> On Mon, Jul 28, 2025 at 3:06 PM Stanislav Fomichev <stfomichev@...il.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?