linux-kernel - Re: Linux 3.3-rc4

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Message-ID: <CA+55aFwBD7pbOcSUVguF+vFAs85JhNpjJgZtDKjEJA0PVXrhkw@mail.gmail.com>
Date:	Mon, 20 Feb 2012 21:06:50 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ian Kent <raven@...maw.net>
Cc:	David Miller <davem@...emloft.net>, thomas@...3r.de,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: Linux 3.3-rc4

On Mon, Feb 20, 2012 at 8:52 PM, Ian Kent <raven@...maw.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.

               Linus
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