netdev - Re: [PATCH] net: Add ndo_gso

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Date:	Tue, 7 Oct 2014 15:05:51 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	Tom Herbert <therbert@...gle.com>, Jesse Gross <jesse@...ira.com>,
	"gerlitz.or@...il.com" <gerlitz.or@...il.com>,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Thomas Graf <tgraf@...g.ch>,
	Pravin Shelar <pshelar@...ira.com>,
	Andy Zhou <azhou@...ira.com>
Subject: Re: [PATCH] net: Add ndo_gso_check

On Tue, Oct 7, 2014 at 1:48 PM, David Miller <davem@...emloft.net> wrote:
>> They're exposing packet parsers to users, so we will be able to
>> program any protocol into the device without reflashing it.
>> Some of the guys are even allowing reprogramming the parser
>> while packets are flowing.
>
> So we have to write new software in _EVERY_ driver to accomodate this.
>
> That makes zero sense either, and it is unneeded complexity in the
> hardware.

I have to agree that for single purpose of csum verify
adding all the complexity around programmable
parsers doesn't make sense.
To me packet parsing is the first step of HW offload,
regardless whether it's flow based or what else this hw can do.

I'd rather see HW vendors provide programmable parser
for any and all protocols instead of them saying:
here is my device X that supports protocols defined
by standards Y and Z.
(which is the case today and it's not pretty)

> COMPLETE works everywhere, on everything, with no driver changes, and
> is so much harder to get wrong.

agree. I'm not against COMPLETE in any way.

> Every protocol specific feature has major downsides whether you choose
> to see them or not.

I surely don't have a preference to a protocol.
I want to see programmable HW that supports any crazy
packet format.

Let's get back to COMPLETE and VMs example,
because I want to see it fixed.
In such case kernel stack in hypervisor will be pulling
encap headers then populating new field in virtio
ring with updated csum and let guest VMs to continue
adjusting that csum, right?
Alternative would be to do inner processing and
csum verification in hypervisor so that packet
can be marked as DATA_VALID, but is it better?
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