linux-kernel - Re: [patch 34/54] Staging: hv: remove STRUCT_PACKED and STRUCT

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Date:	Fri, 24 Jul 2009 14:50:25 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Jörn Engel <joern@...fs.org>
Cc:	Arnd Bergmann <arnd@...db.de>,
	virtualization@...ts.linux-foundation.org,
	Greg Kroah-Hartman <gregkh@...e.de>,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org,
	virtualization@...ts.osdl.org, Sam Ramji <sramji@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Hank Janssen <hjanssen@...rosoft.com>
Subject: Re: [patch 34/54] Staging: hv: remove STRUCT_PACKED and
 STRUCT_ALIGNED defines

On Fri, 24 Jul 2009 23:32:19 +0200
Jörn Engel <joern@...fs.org> wrote:

> On Tue, 21 July 2009 01:46:41 +0200, Arnd Bergmann wrote:
> > On Friday 17 July 2009, Greg Kroah-Hartman wrote:
> > > @@ -43,7 +43,7 @@ typedef struct _RING_BUFFER {
> > >         // volatile u32 InterruptMask;
> > >         // Ring data starts here + RingDataStartOffset !!! DO NOT place any fields below this !!!
> > >      u8         Buffer[0];
> > > -} STRUCT_PACKED RING_BUFFER;
> > > +} __attribute__((packed)) RING_BUFFER;
> > >  
> > 
> > The data structure is actually packed already, the attribute does not make it better
> > and could be removed. We also have __packed as a shortcut for __attribute__((packed)).
> 
> Honestly, I don't know how useful __packed really is.  In a shared
> kernel/userspace header, it is only defined for the kernel.
> 

As I remember, gcc generates worse code for packed structures on many architectures
since it may have to do byte fetchs/recombining to avoid unaligned
accesses.

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