linux-kernel - Re: [RFC net-next v3 2/4] virtio_net: Prepare for NAPI to queue mapping

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Message-ID: <20250127142400.24eca319@kernel.org>
Date: Mon, 27 Jan 2025 14:24:00 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Joe Damato <jdamato@...tly.com>
Cc: Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org,
 gerhard@...leder-embedded.com, leiyang@...hat.com,
 xuanzhuo@...ux.alibaba.com, mkarsten@...terloo.ca, "Michael S. Tsirkin"
 <mst@...hat.com>, Eugenio Pérez <eperezma@...hat.com>,
 Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo Abeni
 <pabeni@...hat.com>, "open list:VIRTIO CORE AND NET DRIVERS"
 <virtualization@...ts.linux.dev>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC net-next v3 2/4] virtio_net: Prepare for NAPI to queue
 mapping

On Mon, 27 Jan 2025 17:07:54 -0500 Joe Damato wrote:
> > Tx NAPIs are one aspect, whether they have ID or not we may want direct
> > access to the struct somewhere in the core, via txq, at some point, and
> > then people may forget the linking has an unintended effect of also
> > changing the netlink attrs. The other aspect is that driver may link
> > queue to a Rx NAPI instance before napi_enable(), so before ID is
> > assigned. Again, we don't want to report ID of 0 in that case.  
> 
> I'm sorry I'm not sure I'm following what you are saying here; I
> think there might be separate threads concurrently and I'm probably
> just confused :)
> 
> I think you are saying that netdev_nl_napi_fill_one should not
> report 0, which I think is fine but probably a separate patch?
> 
> I think, but am not sure, that Jason was asking for guidance on
> TX-only NAPIs and linking them with calls to netif_queue_set_napi.
> It seems that Jason may be suggesting that the driver shouldn't have
> to know that TX-only NAPIs have a NAPI ID of 0 and thus should call
> netif_queue_set_napi for all NAPIs and not have to deal think about
> TX-only NAPIs at all.
> 
> From you've written, Jakub, I think you are suggesting you agree
> with that, but with the caveat that netdev_nl_napi_fill_one should
> not report 0.

Right up to this point.

> Then, one day in the future, if TX-only NAPIs get an ID they will
> magically start to show up.
> 
> Is that right?

Sort of. I was trying to point out corner cases which would also
benefit from netdev_nl_queue_fill_one() being more careful about 
the NAPI IDs it reports. But the conclusion is the same.

> If so, I'll re-spin the RFC to call netif_queue_set_napi for all
> NAPIs in virtio_net, including TX-only NAPIs and see about including
> a patch to tweak netdev_nl_napi_fill_one, if necessary.

netdev_nl_queue_fill_one(), not netdev_nl_napi_fill_one()

Otherwise SG.

After net-next reopens I think the patch to netdev_nl_queue_fill_one()
could be posted separately. There may be drivers out there which already
link Tx NAPIs, we shouldn't delay making the reporting more careful.