netdev - Re: [patch net-next] virtio_net: add support for Byte Queue Limits

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Date: Mon, 10 Jun 2024 10:18:56 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Jiri Pirko <jiri@...nulli.us>
Cc: Jason Wang <jasowang@...hat.com>,
	Jason Xing <kerneljasonxing@...il.com>,
	Heng Qi <hengqi@...ux.alibaba.com>, davem@...emloft.net,
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
	xuanzhuo@...ux.alibaba.com, virtualization@...ts.linux.dev,
	ast@...nel.org, daniel@...earbox.net, hawk@...nel.org,
	john.fastabend@...il.com, netdev@...r.kernel.org
Subject: Re: [patch net-next] virtio_net: add support for Byte Queue Limits

On Fri, Jun 07, 2024 at 01:30:34PM +0200, Jiri Pirko wrote:
> Fri, Jun 07, 2024 at 12:23:37PM CEST, mst@...hat.com wrote:
> >On Fri, Jun 07, 2024 at 11:57:37AM +0200, Jiri Pirko wrote:
> >> >True. Personally, I would like to just drop orphan mode. But I'm not
> >> >sure others are happy with this.
> >> 
> >> How about to do it other way around. I will take a stab at sending patch
> >> removing it. If anyone is against and has solid data to prove orphan
> >> mode is needed, let them provide those.
> >
> >Break it with no warning and see if anyone complains?
> 
> This is now what I suggested at all.
> 
> >No, this is not how we handle userspace compatibility, normally.
> 
> Sure.
> 
> Again:
> 
> I would send orphan removal patch containing:
> 1) no module options removal. Warn if someone sets it up
> 2) module option to disable napi is ignored
> 3) orphan mode is removed from code
> 
> There is no breakage. Only, hypotetically performance downgrade in some
> hypotetical usecase nobody knows of.

Performance is why people use virtio. It's as much a breakage as any
other bug. The main difference is, with other types of breakage, they
are typically binary and we can not tolerate them at all.  A tiny,
negligeable performance regression might be tolarable if it brings
other benefits. I very much doubt avoiding interrupts is
negligeable though. And making code simpler isn't a big benefit,
users do not care.

> My point was, if someone presents
> solid data to prove orphan is needed during the patch review, let's toss
> out the patch.
> 
> Makes sense?

It's not hypothetical - if anything, it's hypothetical that performance
does not regress.  And we just got a report from users that see a
regression without.  So, not really.

> 
> >
> >-- 
> >MST
> >