netdev - Re: [PATCH net-next 2/2] selftests: drv-net: gro: run the test against HW GRO and LRO

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Message-ID: <20251201115041.5aa4c986@kernel.org>
Date: Mon, 1 Dec 2025 11:50:41 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: davem@...emloft.net, netdev@...r.kernel.org, edumazet@...gle.com,
 pabeni@...hat.com, andrew+netdev@...n.ch, horms@...nel.org,
 shuah@...nel.org, sdf@...ichev.me, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net-next 2/2] selftests: drv-net: gro: run the test
 against HW GRO and LRO

On Sun, 30 Nov 2025 09:56:24 -0500 Willem de Bruijn wrote:
> Jakub Kicinski wrote:
> > On Fri, 28 Nov 2025 15:42:40 -0500 Willem de Bruijn wrote:  
> > > So GRO off disables HW_GRO, but not LRO? That difference is behavior
> > > is confusing. Could we still see this as a regression and make the
> > > ethtool HW_GRO feature equally independent from SW_GRO?  
> > 
> > I couldn't convince myself that it's justified. Of course it would have
> > made testing a lot easier. But apart from that - what's your reading of
> > the status quo? Working backwards from were we ended up (and I
> > haven't dug into the git history) I'm guessing that LRO disable is used
> > to prevent changing geometry of the packets. GRO would presumably be
> > disabled when user knows that it will be ineffective, to save the cost.
> > Or when some portion of the stack (XDP?) can't deal with super frames.
> > 
> > If those are the reasons, practically, I don't see why user would want
> > HW GRO without SW. Ever since we allowed SW GRO to re-GRO HW GRO'ed
> > frames it's always better to leave SW enabled. HW leaves a lot of
> > aggregation opportunities on the table.
> > 
> > I concluded that changing the current behavior would not help any real
> > life scenario, just testing. LMK if you see one or the inconsistency
> > is a big enough reason.  
> 
> I think that's fair.
> 
> But from reading the code I don't see how disabling NETIF_F_GRO also
> disables NETIF_F_GRO_HW. And indeed I just tested on one (admittedly
> not latest upstream) IDPF driver and it does not.

Looks like you're right. Broadcom drivers where GRO_HW originates do it
locally, so does qede. I guess somewhere along the way drives started
treating GRO_HW as a separate feature rather than a GRO offload. 

I don't think it changes the reasoning in any major way?