linux-kernel - Re: [PATCH v2 03/44] Add CONFIG_HAVE_64BIT_ALIGNED

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Message-ID: <CAMuHMdXpw+hWgcqDiFVdDTsFBEThjvB_Y0jcoZZHS-JGTBvYRg@mail.gmail.com>
Date:	Mon, 10 Dec 2012 13:55:39 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	James Hogan <james.hogan@...tec.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	Balbir Singh <bsingharora@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Paris <eparis@...hat.com>, Will Drewry <wad@...omium.org>,
	James Morris <james.l.morris@...cle.com>
Subject: Re: [PATCH v2 03/44] Add CONFIG_HAVE_64BIT_ALIGNED_STRUCT for taskstats

On Mon, Dec 10, 2012 at 11:22 AM, James Hogan <james.hogan@...tec.com> wrote:
> On 08/12/12 03:43, H. Peter Anvin wrote:
>> On 12/05/2012 08:08 AM, James Hogan wrote:
>>> On 64 bit architectures with no efficient unaligned access, taskstats
>>> has to add some padding to a reply to prevent unaligned access warnings.
>>> However this also needs to apply to 32 bit architectures with 64 bit
>>> struct alignment such as metag (which has 64 bit memory accesses).
>>
>> Wait... 64-bit struct alignment on structures with only 32-bit members?
>>  That might be... interesting... in a number of places...
>
> I'll rewrite the description as it's a bit misleading. On metag 64bit
> struct alignment is required when it contains 64bit members, not if it
> only contains 32bit members. Although metag is a 32bit arch, it can do
> 64bit memory accesses which must be aligned.

The C alignment rules should take care of this automatically (struct alignment
is the maximum alignment of its members).

You only have to override this manually in cases like this, where
64-bit quantities
are stored in char arrays:

 struct buffer_data_page {
        u64              time_stamp;    /* page time stamp */
        local_t          commit;        /* write committed index */
-       unsigned char    data[];        /* data of buffer page */
+       unsigned char    data[] RB_ALIGN_DATA;  /* data of buffer page */
 };

(cfr. your other patch).

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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