netdev - Re: [PATCH v1 1/6] net: Generalize udp based tunnel offload

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Date:	Tue, 1 Dec 2015 19:50:17 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	"John W. Linville" <linville@...driver.com>,
	Jesse Gross <jesse@...nel.org>,
	David Miller <davem@...emloft.net>,
	Anjali Singhai Jain <anjali.singhai@...el.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Kiran Patil <kiran.patil@...el.com>
Subject: Re: [PATCH v1 1/6] net: Generalize udp based tunnel offload

On Tue, Dec 1, 2015 at 7:49 AM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> On Tue, Dec 1, 2015, at 16:44, John W. Linville wrote:
>> On Mon, Nov 30, 2015 at 09:26:51PM -0800, Tom Herbert wrote:
>> > On Mon, Nov 30, 2015 at 5:28 PM, Jesse Gross <jesse@...nel.org> wrote:
>>
>> > > Based on what we can do today, I see only two real choices: do some
>> > > refactoring to clean up the stack a bit or remove the existing VXLAN
>> > > offloading altogether. I think this series is trying to do the former
>> > > and the result is that the stack is cleaner after than before. That
>> > > seems like a good thing.
>> >
>> > There is a third choice which is to do nothing. Creating an
>> > infrastructure that claims to "Generalize udp based tunnel offload"
>> > but actually doesn't generalize the mechanism is nothing more than
>> > window dressing-- this does nothing to help with the VXLAN to
>> > VXLAN-GPE transition for instance. If geneve specific offload is
>> > really needed now then that can be should with another ndo function,
>> > or alternatively ntuple filter with a device specific action would at
>> > least get the stack out of needing to be concerned with that.
>> > Regardless, we will work optimize the rest of the stack for devices
>> > that implement protocol agnostic mechanisms.
>>
>> Is there no concern about NDO proliferation? Does the size of the
>> netdev_ops structure matter? Beyond that, I can see how a single
>> entry point with an enum specifying the offload type isn't really any
>> different in the grand scheme of things than having multiple NDOs,
>> one per offload.
>>
>> Given the need to live with existing hardware offloads, I would lean
>> toward a consolidated NDO. But if a different NDO per tunnel type is
>> preferred, I can be satisified with that.
>
> Having per-offloading NDOs helps the stack to gather further information
> what kind of offloads the driver has even maybe without trying to call
> down into the layer (just by comparing to NULL). Checking this inside
> the driver offload function clearly does not have this feature. So we
> finally can have "ip tunnel please-recommend-type" feature. :)
>
That completely misses the whole point of the rest of this thread.
Protocol specific offloads are what we are trying to discourage not
encourage. Adding any more ndo functions for this purpose should be an
exception, not the norm. The bar should be naturally high considering
the cost of exposing this to ndo.
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