netdev - Re: [PATCH net-next v2 1/5] net: Introduce NETIF_F_GRO

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Message-ID: <CACKFLimna4yO28h8JAVSeqdSOtnQJKdVdKHbZiMxsngj65=gLg@mail.gmail.com>
Date:   Thu, 7 Dec 2017 16:05:51 -0800
From:   Michael Chan <michael.chan@...adcom.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>,
        Andrew Gospodarek <andrew.gospodarek@...adcom.com>,
        Ariel Elior <Ariel.Elior@...ium.com>,
        everest-linux-l2@...ium.com
Subject: Re: [PATCH net-next v2 1/5] net: Introduce NETIF_F_GRO_HW.

On Thu, Dec 7, 2017 at 3:35 PM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
> On Thu, Dec 7, 2017 at 3:17 PM, Michael Chan <michael.chan@...adcom.com> wrote:
>> I don't get this.  I don't see how TSO is related.
>
> It isn't. That is the point. If I change ANY feature it will trigger
> fix_features to get called. It will then see we have GRO_HW and LRO
> since we have one interface that supports one and one that supports
> another. It will trigger this code and cause LRO to be disabled which
> then will be propagated down.

I see.  But this won't happen.  Because the bonding driver is not
advertising NETIF_F_GRO_HW in its hw_features.  It is not advertising
NETIF_F_GRO either but I think it gets added automatically since it is
a software feature.  So LRO won't get disabled on the bond when a
unrelated feature is changed.

But I think I see your point.  I can make it so that it is up to
individual driver's .ndo_fix_features() to drop LRO/GRO_HW as it sees
fit, instead of doing it in the common netdev_fix_features().  That
way, it is more flexible at least.

>
>>> That
>>> is a bad behavior and shouldn't happen. In addition I would want to be
>>> able to disable GRO_HW from the bond should I encounter a buggy
>>> implementation of the GRO hardware offload at some point in the
>>> future. I don't like it being tied so closely with GRO since all it
>>> takes is one hardware failure and then we are dealing with people
>>> disabling GRO due to reports of it being buggy instead of it being
>>> tied specifically to GRO_HW.
>>>
>>
>> Today, GRO, RXCSUM, NTUPLE, etc on a bond do not get propagated.  You
>> can propose and make them propagate if you want.
>
> Or you can fix your patches so you take it into account now instead of
> putting things in a broken state and asking others to fix it.

I don't think that things are necessarily broken today.  LRO truly
needs to be propagated.  It's debatable whether other features like
GRO/RXCSUM/NTUPLE should be centrally set by the upper device or not.

>
>> So GRO_HW just naturally follows GRO since it is a true subset of it.
>
> Right, and I am saying you have exposed a bug. If I don't want GRO on
> the bond it should be propagated down. It doesn't make sense to allow
> lower devices to assemble frames when the bond doesn't want them and
> GSO won't kick in before it gets to the bond.

GRO kicks in at the lower device before it gets to the bond if the
lower device calls napi_gro_receive() and GRO is enabled.