Re: [PATCH net-next v3] net: dqs: introduce IFF_NO_BQL private flag for non-BQL drivers

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jun 13, 2024 at 09:51:00AM +0200, Jiri Pirko wrote:
> Thu, Jun 13, 2024 at 09:24:27AM CEST, kerneljasonxing@gmail.com wrote:
> >On Thu, Jun 13, 2024 at 3:19 PM Jiri Pirko <jiri@resnulli.us> wrote:
> >>
> >> Thu, Jun 13, 2024 at 08:08:36AM CEST, kerneljasonxing@gmail.com wrote:
> >> >On Thu, Jun 13, 2024 at 1:38 PM Jiri Pirko <jiri@resnulli.us> wrote:
> >> >>
> >> >> Thu, Jun 13, 2024 at 04:35:49AM CEST, kerneljasonxing@gmail.com wrote:
> >> >> >From: Jason Xing <kernelxing@tencent.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(). Let me give it a
> >> >> >try first.
> >> >> >
> >> >> >I decided to introduce a NO_BQL bit 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 in one go.
> >> >> >
> >> >> >As Jakub said, "netdev_uses_bql() is best effort", I think, we can add
> >> >> >new non-BQL drivers as soon as we find one.
> >> >> >
> >> >> >After this patch, there is no byte_queue_limits directory in virtio-net
> >> >> >driver.
> >> >>
> >> >> Please note following patch is currently trying to push bql support for
> >> >> virtio_net:
> >> >> https://lore.kernel.org/netdev/20240612170851.1004604-1-jiri@resnulli.us/
> >> >
> >> >I saw this one this morning and I'm reviewing/testing it.
> >> >
> >> >>
> >> >> When that is merged, this patch is not needed. Could we wait?
> >> >
> >> >Please note this patch is not only written for virtio_net driver.
> >> >Virtio_net driver is one of possible cases.
> >>
> >> Yeah, but without virtio_net, there will be no users. What's the point
> >> of having that in code? I mean, in general, no-user kernel code gets
> >> removed.
> >
> >Are you sure netdev_uses_bql() can limit all the non-bql drivers with
> >those two checks? I haven't investigated this part.
> 
> Nope. What I say is, if there are other users, let's find them and let
> them use what you are introducing here. Otherwise don't add unused code.


Additionally, it looks like virtio is going to become a
"sometimes BQL sometimes no-BQL" driver, so what's the plan -
to set/clear the flag accordingly then? What kind of locking
will be needed?

> >
> >>
> >>
> >> >
> >> >After your patch gets merged (I think it will take some time), you
> >> >could simply remove that one line in virtio_net.c.
> >> >
> >> >Thanks.