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

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Message-ID: <20090724230619.GB13702@logfs.org>
Date:	Sat, 25 Jul 2009 01:06:20 +0200
From:	Jörn Engel <joern@...fs.org>
To:	Stephen Hemminger <shemminger@...ux-foundation.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 July 2009 14:50:25 -0700, Stephen Hemminger wrote:
> 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:
> > > 
> > > 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.

I was talking about "__packed" vs. "__attribute__((packed))".  But yes,
avoiding packed structures where possible is a good idea.

Jörn

-- 
"[One] doesn't need to know [...] how to cause a headache in order
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-- Scott Culp, Manager of the Microsoft Security Response Center, 2001
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