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

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Date: Mon, 10 Jun 2024 07:55:55 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Eric Dumazet <edumazet@...gle.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 Mon, Jun 10, 2024 at 12:42 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> 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)

Thanks for your reminder. When I was trying to introduce one new bit,
I noticed an overflow warning when compiling.

>
> 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.

The reason is that I just followed the comment to force myself to
report to ethtool. Now I see.

It seems not that easy to consider all the non-BQL drivers. Let me
think more about it.

Thanks,
Jason