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

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Message-ID: <Z5gJYOAUVZH8-4Pt@LQ3V64L9R2>
Date: Mon, 27 Jan 2025 17:32:00 -0500
From: Joe Damato <jdamato@...tly.com>
To: Jakub Kicinski <kuba@...nel.org>
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, Jan 27, 2025 at 02:24:00PM -0800, Jakub Kicinski wrote:
> 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()

Right, sorry for the typo/added confusion.
 
> 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.

OK, I'll start with that when net-next reopens while waiting on the
locking changes to come later and do the actual linking.