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

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Message-ID: <aIo_RMVBBWOJ7anV@mini-arch>
Date: Wed, 30 Jul 2025 08:50:28 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: 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,
	almasrymina@...gle.com, 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 07/30, Pavel Begunkov wrote:
> On 7/29/25 17:33, Stanislav Fomichev wrote:
> > On 07/28, Pavel Begunkov wrote:
> > > On 7/28/25 23:06, Stanislav Fomichev 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
> > > 
> > > Could be needed, but there are cases where configuration and
> > > virtual queue selection is done outside the program. I'll need
> > > to ask which option we currently use.
> > 
> > If the setup is done outside, you can also setup rx-buf-len outside, no?
> 
> You can't do it without assuming the memory layout, and that's
> the application's role to allocate buffers. Not to mention that
> often the app won't know about all specifics either and it'd be
> resolved on zcrx registration.

I think, fundamentally, we need to distinguish:

1. chunk size of the memory pool (page pool order, niov size)
2. chunk size of the rx queue entries (this is what this series calls
   rx-buf-len), mostly influenced by MTU?

For devmem (and same for iou?), we want an option to derive (2) from (1):
page pools with larger chunks need to generate larger rx entries.