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

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Date:	Wed, 7 Jan 2015 10:03:30 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Alexei Starovoitov' <alexei.starovoitov@...il.com>,
	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

From: Alexei Starovoitov
> 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.

Also, if you are writing the values then you need to write
all the members of the bitfield in order to get a single write.

Basically you are much better off using explicit masks.

	David