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Subject: Re: Linux 3.3-rc4
From: Ian Kent <raven@themaw.net>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: David Miller <davem@davemloft.net>, thomas@m3y3r.de,
        linux-kernel@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>
Date: Tue, 21 Feb 2012 13:39:47 +0800
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On Mon, 2012-02-20 at 21:06 -0800, Linus Torvalds wrote:
> On Mon, Feb 20, 2012 at 8:52 PM, Ian Kent <raven@themaw.net> wrote:
> >
> > I think the patch attached to your original post needs a little work if
> > that is to be used. Correct me if I'm wrong but AFAICT there are more
> > architectures that use 8-byte alignment than just x86-64, such as alpha,
> > ia64 and ppc64 and I believe they may also be used in a compat mode.
> 
> The only issue is compat mode, and afaik, all other architectures
> except for x86-32 do __u64 with natural alignment.
> 
> So all 64-bit architectures use natural alignment, the only issue is
> the alignment of __u64 in 32-bit mode.
> 
> So it really is *not* about 8-byte alignment. Quite the reverse. It's
> about 4-byte alignment of 64-bit entities, and I suspect x86-32 is the
> only one that does that.
> 
> See "compat_u64", and notice how only in arch/x86/include/asm/compat.h
> do we have
> 
>   typedef u64 __attribute__((aligned(4))) compat_u64;
> 
> So it really is limited to only x86.

Thanks, I wasn't aware of this.

> 
>                Linus