Date:   Thu, 7 Dec 2017 20:02:19 -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 6:36 PM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
> On Thu, Dec 7, 2017 at 4:05 PM, Michael Chan <michael.chan@...adcom.com> wrote:
>> 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.
>
> Thank you.

OK.  I will make this change for V3.

>>
>> 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 I can agree with the NTUPLE not being propagated since it doesn't
> actually effect upper devices. Really the functionality only really
> has effects locally since the functionality consists of route to a
> specific queue/device or drop the packet.
>
> I'm not sure why RXCSUM isn't being propagated. It seems like that is
> something that would make sense to have passed all the way down to the
> lower devices since a single device that is doing bad Rx checksum
> offloads could potentially corrupt all traffic in a bond. Seems like
> that one should definitely be included.

This is a separate discussion that goes beyond GRO.  There are pros
and cons for the upper device to propagate every single feature flag.

>>
>> 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.
>
> I get that. I assume the reason why the bond doesn't have it enabled
> is because we don't want it to kick in at every given netdev, there
> isn't any point to do GRO more than once. The problem is GRO_HW isn't
> a pure software offload like GRO is. Call me a pessimist, but when we
> end up encountering a buggy implementation that has to be disabled we
> will want the right infrastructure in place to handle it. It becomes
> another argument for why we might want to split GRO_HW and GRO without
> tying them together. It would make sense to expose GRO_HW in a bond,
> but not GRO. It might be something where we want to do any close tying
> together of the GRO flag and GRO_HW at the driver as well. Basically
> the legacy devices that transition over to GRO_HW from using just the
> GRO flag could do that to maintain existing functionality, and new
> drivers that implement it could opt in to the same behavior or just
> handle GRO_HW as a separate flag.

To me, making GRO_HW dependent on GRO makes the most intuitive sense.
Separating them is just confusing.  The possibility of GRO_HW being
enabled without GRO enabled makes no sense to me.

>
> Actually I just had a thought. What if we consider this a separate GRO
> stage instead of just a hardware offload? Our standard GRO is a post
> receive from the driver perspective, basically the packet is assembled
> after we have handed it to the stack. What you are doing with GRO_HW
> is essentially providing an early reassembly before it is handed to
> the stack. What if we were to rename GRO_HW to something like
> GRO_LOWER, GRO_EARLY, GRO_PRE, or pick your name (I'm lousy at
> naming), and used it as a way to indicate that we want to perform GRO
> before we begin receive processing on the frame in our driver? Then
> for stacked devices you could use this new flag to indicate you don't
> want to perform GRO on the lower levels below this device, and could
> then use the regular GRO flag to control if we do it ourselves. Doing
> that should provide stacked devices with a good way to control GRO on
> the lower devices and would resolve what you need to indicate as well.
> The only real changes needed might be a rename and to add the
> necessary bit shifting for the upper and lower dev sync code. If you
> aren't interested in the idea I can probably spend a couple of hours
> getting to it tomorrow since I think this might be a much better way
> to go as it solves multiple issues.

I really don't see what a different name will buy us.  If you want to
propagate GRO/GRO_HW, we can do that if others agree.  I only feel
strongly that GRO/GRO_HW should be tied.  I don't feel strongly
whether GRO/GRO_HW should be propagated or not propagated.  Again, LRO
needs to be propagated out of necessity (e.g. when a bond is added to
a bridge).

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