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

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Date:	Tue, 6 Jan 2015 19:37:15 -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 Tue, Jan 6, 2015 at 6:05 PM, Thomas Graf <tgraf@...g.ch> wrote:
> +struct vxlan_gbp {
> +#ifdef __LITTLE_ENDIAN_BITFIELD
> +       __u8    reserved_flags1:3,
...
> +       __be16 policy_id;
> +} __packed;

are you sure that compiler will be smart enough
to do single short load when you pack
u8 + struct vxlan_gbp inside struct vxlanhdr ?
I suspect compiler will use two byte loads
with shifts and ors every time to access policy_id.
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.
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