netdev - Re: [PATCH net-next] net: dqs: introduce NETIF_F_NO

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Message-ID: <CANn89iK+UWubgdKYd3g7Q+UjibDqUD+Lv5kfmEpB+Rc0SxKT6w@mail.gmail.com>
Date: Sun, 9 Jun 2024 18:42:36 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: kuba@...nel.org, pabeni@...hat.com, davem@...emloft.net, 
	dsahern@...nel.org, mst@...hat.com, jasowang@...hat.com, 
	xuanzhuo@...ux.alibaba.com, eperezma@...hat.com, leitao@...ian.org, 
	netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next] net: dqs: introduce NETIF_F_NO_BQL device feature

On Sun, Jun 9, 2024 at 3:17 PM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> From: Jason Xing <kernelxing@...cent.com>
>
> Since commit 74293ea1c4db6 ("net: sysfs: Do not create sysfs for non
> BQL device") limits the non-BQL driver not creating byte_queue_limits
> directory, I found there is one exception, namely, virtio-net driver,
> which should also be limited in netdev_uses_bql().
>
> I decided to introduce a NO_BQL bit in device feature because
> 1) it can help us limit virtio-net driver for now.
> 2) if we found another non-BQL driver, we can take it into account.
> 3) we can replace all the driver meeting those two statements in
> netdev_uses_bql() in future.
>
> For now, I would like to make the first step to use this new bit for dqs
> use instead of replacing/applying all the non-BQL drivers.
>
> After this patch, 1) there is no byte_queue_limits directory in virtio-net
> driver. 2) running ethtool -k eth1 shows "no-bql: on [fixed]".
>
> Signed-off-by: Jason Xing <kernelxing@...cent.com>

I do not think we want to consume a precious bit from dev->features
for something like that.

dev->features should be reserved for bits we used in the fast path, for instance
netif_skb_features(). It is good to have free bits for future fast path use.

(I think Vladimir was trying to make some room, this was a discussion
we had last year)

I do not see the reason to report to ethtool the 'nobql bit' :
If a driver opts-out, then the bql sysfs files will not be there, user
space can see the absence of the files.