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

[Joe Damato <jdamato@fastly.com>]

On Mon, Jan 27, 2025 at 01:33:04PM -0800, Jakub Kicinski wrote:
> On Mon, 27 Jan 2025 14:31:21 -0500 Joe Damato wrote:
> > Actually, I missed a patch Jakub submit to net [1], which prevents
> > dumping TX-only NAPIs.
> 
> That patch only addresses NAPI ops, here I think we're talking about
> attributes of the queue object.

Yea, that's true.

> > So, I think this RFC as-is (only calling netif_queue_set_napi
> > for RX NAPIs) should be fine without changes.
> 
> Weak preference towards making netdev_nl_queue_fill_one() "do the right
> thing" when NAPI does not have ID assigned. And right thing IMO would
> be to skip reporting the NAPI_ID attribute.

Ah, right. Your patch was for netdev_nl_napi_fill_one, so presumably
a similar patch would need to be written for
netdev_nl_queue_fill_one.

> 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.

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

Is that right?

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.