netdev - Re: [PATCH 2/6] vxlan: Group Policy extension

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Date:	Wed, 7 Jan 2015 09:32:52 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Thomas Graf <tgraf@...g.ch>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Jesse Gross <jesse@...ira.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Pravin Shelar <pshelar@...ira.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"dev@...nvswitch.org" <dev@...nvswitch.org>
Subject: Re: [PATCH 2/6] vxlan: Group Policy extension

On Wed, Jan 7, 2015 at 3:10 AM, Thomas Graf <tgraf@...g.ch> wrote:
> On 01/06/15 at 07:37pm, Alexei Starovoitov wrote:
>> Even it works ok, I think this struct layout is ugly.
>> imo would be much easier to read if you replace
>> the whole vxlanhdr with vxlanhdr_gbp
>> or split vxlanhdr into two 32-bit structs.
>> then __packed hacks won't be needed.
>
> The main reason why I merged it into vxlanhdr is for documentation
> purposes and to avoid duplicating the generic VXLAN header for every
> extension. The RCO and GPE extensions would need to duplicate this
> over and over. It gets messy in particular when multiple extensions
> can be used in combination (such as GBP and RCO) which then each
> have their own conflicting header definitions. This way, it is clear
> which extensions are compatible by just looking at the definition
> of the structure.

I'm afraid 'union' style with first u8 flags working as selector
won't work for the case you're describing, but since
                      md.gbp = ntohs(vxh->gbp.policy_id);
    2652:       41 0f b7 55 0a          movzwl 0xa(%r13),%edx
then at least from performance side it's ok at least on x86.
So this _packed stuff is fine, though not pretty.
It's internal header, so we can improve it later.
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