Message-ID: <46fabfb5-ee39-43a2-986e-30df2e4d13ab@gmail.com>
Date: Wed, 30 Jul 2025 15:16:30 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Stanislav Fomichev <stfomichev@...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 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.

>>> steering? And if we end up adding queue api, you'll have to call that
>>> one over netlink also.
>>
>> There is already a queue api, even though it's cropped IIUC.
>> What kind of extra setup you have in mind?
> 
> I'm talking about allocating the queues. Currently the zc/devmem setup is
> a bit complicated, we need to partition the queues and rss+flow
> steer into a subset of zerocopy ones. In the future we might add some apis
> to request a new dedicated queue for the specific flow(s). That should
> hopefully simplify the design (and make the cleanup of the queues more
> robust if the application dies).

I see, would be useful indeed, but let's not over complicate things
until we have to, especially since there are reasons not to. For
the configuration, I was arguing for a while that it'd be great to
have an allocated queue wrapped into an fd, so that all
containerisation / queue passing / security / etc. questions just
solved in a generic and ubiquitous way.

>>>>> If we assume that at some point niov can be backed up by chunks larger
>>>>> than PAGE_SIZE, the assumed workflow for devemem is:
>>>>> 1. change rx-buf-len to 32K
>>>>>      - this is needed only for devmem, but not for CPU RAM, but we'll have
>>>>>        to refill the queues from the main memory anyway
>>>>
>>>> Urgh, that's another reason why I prefer to just pass it through
>>>> zcrx and not netlink. So maybe you can just pass the len to devmem
>>>> on creation, and internally it sets up its queues with it.
>>>
>>> But you still need to solve MAX_PAGE_ORDER/PAGE_ALLOC_COSTLY_ORDER I
>>> think? We don't want the drivers to do PAGE_ALLOC_COSTLY_ORDER costly
>>> allocation presumably?
>>
>> #define PAGE_ALLOC_COSTLY_ORDER 3
>>
>> It's "costly" for the page allocator and not a custom specially
>> cooked memory providers. Nobody should care as long as the length
>> applies to the given provider only. MAX_PAGE_ORDER also seems to
>> be a page allocator thing.
> 
> By custom memory providers you mean page pool? Thinking about it more,

zcrx / devmem. I'm just saying that in situations where zcrx sets
the size for its queues and that only affects zcrx allocations and
not normal page pools, PAGE_ALLOC_COSTLY_ORDER is irrelevant.

I agreed on dropping the netlink queue size set, which leaves the
global size set, but that's a separate topic.

> maybe it's fine as is as long as we have ndo_queue_cfg_validate that
> enforces sensible ranges..

-- 
Pavel Begunkov

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