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

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Message-ID: <50CEEB08.1080707@imgtec.com>
Date:	Mon, 17 Dec 2012 09:51:04 +0000
From:	James Hogan <james.hogan@...tec.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
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 10/12/12 12:55, Geert Uytterhoeven wrote:
> 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).

Hi Geert,

Please see the comment in mk_reply in kernel/taskstats.c. The structure
is being serialised after 2 NLA headers and a pid which is why the extra
padding needs to be added manually.

Cheers
James

> 
> 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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