Date:   Sat, 9 Dec 2017 22:40:40 -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 v3 1/5] net: Introduce NETIF_F_GRO_HW.

On Sat, Dec 9, 2017 at 2:04 PM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
> On Sat, Dec 9, 2017 at 1:31 PM, Michael Chan <michael.chan@...adcom.com> wrote:
>> On Sat, Dec 9, 2017 at 10:50 AM, Alexander Duyck
>> <alexander.duyck@...il.com> wrote:
>>> So I would disagree with it being a subset of NETIF_F_GRO. If anything
>>> it is an alternative to NETIF_F_GRO. It is performing GRO much earlier
>>> at the device level in the case of hardware drivers. My concern is
>>> this is probably going to end up applying to things other than just
>>> hardware drivers though. For example what is to prevent this from
>>> being applied to something like a virtio/tap interface? It seems like
>>> this should be something that would be easy to implement in software.
>>
>> If you do it in software, it's called NETIF_F_GRO.  We already have
>> it.  The whole point of the new flag is that if the device has
>> software GRO enabled, and if the device supports GRO_HW, then we can
>> do a subset of GRO in hardware (hopefully faster).
>
> I can see what you are getting at. But GRO_HW with GRO stacked on top
> of it won't necessarily be the fastest form of GRO. If you have a
> GRO_HW implementation that is complete enough people may want to
> disable Software GRO in order to avoid the extra overhead involved
> with using it.

It is possible that if you have incoming packets 1, 2, 3, 4, 5 for a
TCP connection, HW_GRO can aggregate packets 1, 2, 3, but cannot
aggregate packets 4 and 5 due to hardware resource limitation.
Software GRO aggregates 4 and 5.  So it works well together.

>>> I'm going to back off on my requirement for you to handle propagation
>>> since after spending a couple hours working on it I did find it was
>>> more complex then I originally thought it would be. With that said
>>> however I would want to see this feature implemented in such a way
>>> that we can deal with propagating the bits in the future if we need to
>>> and that is what I am basing my comments on.
>>
>> Nothing stops anyone from propagating the flag.  Just add
>> NETIF_F_GRO_HW to NETIF_F_UPPER_DISABLES and it will be propagated
>> just like LRO.
>
> Yes, but the problem then is it doesn't solve the secondary issue of
> no way to propagate down the desire to disable GRO as well. That is
> why I am thinking that the new bit could be used to indicate that we
> want GRO to be supported either in the driver or below it instead of
> only in "hardware". We are much better off with a generic solution and
> that is why I think it might be better to use more of a pipeline or
> staged type definition for this. Basically with GRO it occurs in the
> GRO logic just after the driver hands off the packet, while this new
> bit indicates that GRO happens somewhere before then. If we use that
> definition for this then it becomes usable to deal with things like
> the stacked devices problem where the stacked devices normally have
> the GRO flag disabled since we don't want to run GRO multiple times,
> but as a result the stacked devices have no way of saying they don't
> want GRO. If we tweak the definition of this bit it solves that
> problem since it would allow for us disabling GRO, GRO_HW, and LRO on
> any devices below a given device.

I just don't follow your logic.  First of all, GRO on an upper device
doesn't mean that we are doing GRO on the upper device.  The bonding
driver cannot do GRO because it doesn't call napi_gro_receive().  GRO
always happens on the lower device.  Propagation of GRO can only mean
that if GRO is set on the upper device, GRO is propagated and allowed
on lower devices.  Nothing stops you from doing that if you want to do
that.

>>> I still disagree with this bit. I think GRO is a pure software
>>> offload, whereas GRO_HW can represent either a software offload of
>>> some sort occurring in or before the driver, or in the hardware.
>>> Basically the difference between the two as I view it is where the GRO
>>> is occurring. I would like to keep that distinction and make use of
>>> it. As I mentioned before in the case of bonding we currently have no
>>> way to disable GRO on the lower devices partially because GRO is a
>>> pure software feature and always happens at each device along the way.
>>> The nice thing about this new bit is the assumption is that it is
>>> pushing GRO to the lowest possible level and not triggering any side
>>> effects like GRO currently does. I hope to use that logic with stacked
>>> devices so that we could clear the bit and have it disable GRO,
>>> GRO_HW, and LRO on all devices below the device that cleared it.
>>>
>>> I think this linking of GRO and GRO_HW is something that would be
>>> better served by moving it into the driver if you are wanting to
>>> maintain the behavior of how this was previously linked to GRO.
>>
>> If you insist, I can move this to the driver's ndo_fix_features().
>> But I feel it is much better to enforce this dependency system wide.
>> Once again, GRO_HW is hardware accelerated GRO and should depend on
>> it.
>
> The question I would have is why? Where is the dependency? I don't see
> it. It is GRO in one spot and/or GRO in the other. The two don't
> interract directly and I don't believe you can do software GRO on a
> frame that has already been coalesced in hardware,

Right.  But hardware can do a series of frames and software can do a
different series of frames that have not been aggregated.

>> This is a logical feature dependency that Yuval Mintz suggested.  For
>> GRO_HW to work, hardware must verify the checksum of a packet before
>> the packet can be merged.
>>
>> So if the user does not want to do RXCSUM on this device for whatever
>> reason, it logically means that he also doesn't want to do GRO_HW with
>> implied RXCSUM performed on each packet that is merged.
>>
>> So I agree with Yuval that this dependency makes sense.
>
> Okay then, so if we are going to go that route we may want to be
> complete on this and just disable GRO_HW and LRO if RXCSUM is not
> enabled. We might also want to add a comment indicating that we don't
> support anything that might mangle a packet at the driver level if
> RXCSUM is not enabled. Comments explaining all this would be a good
> thing just to keep someone from grabbing GRO and lumping it in at some
> point in the future.
>
> I'm still working on trying to propagate the Rx checksum properly
> since it should probably follow the same UPPER_DISABLES behavior as
> LRO, but I will probably only have a few hours over the next week to
> really work on any code and there end up being a number of stacked
> drivers that have to be updated. I would be good with just flipping
> this logic for now and if RXCSUM is not set, and GRO_HW (just noticed
> the typo in your message) is set, then print your message and clear
> the bit. I can probably come back later and add LRO once I get the
> propagation bits worked out.

Just fix the netdev_dbg() typo, right?  I don't understand what you
mean by flipping the logic.  It's the same whether you check RXCSUM
first or GRO_HW first.

May be you meant put the RXCSUM check in the outer if statement so
that someone could add more inner checks?  OK, I think that's what you
meant.

>
> As far as patch 2 in the set it would probably be better to either
> drop it and just accept it as an outstanding issue, or you could take
> on the propagation problems with GRO_HW and RXCSUM since we really
> need to get those solved in order for this functionality to fully
> work.

We need patch #2 otherwise generic GRO won't work on these 3 drivers.
I don't think I fully understand your concerns about propagation.  To
me propagation is just a usage model where an upper device will
control the common features of lower devices.  It is more convenient
to have propagation, but requires upper devices to be aware of all
features that propagate (GRO, RXCSUM).  Without propagation, it is
still fine.

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