Re: CVS commit: src/sys/sys

[David Laight <david%l8s.co.uk@localhost>]

On Mon, Sep 15, 2014 at 11:58:35AM +0900, NONAKA Kimihiro wrote:
> Hi,
> 
> 2014-09-15 3:07 GMT+09:00 Christos Zoulas <christos%astron.com@localhost>:
> 
> > Perhaps it is better to memcpy the data to some aligned struct rather
> > than referencing it directly and making it packed (which is not portable).
> 
> I wrote the following patch. ok?
> 
> Index: ./sys/sys/bootblock.h
> ===================================================================
> RCS file: /cvsroot/src/sys/sys/bootblock.h,v
> retrieving revision 1.57
> diff -u -r1.57 bootblock.h
> --- ./sys/sys/bootblock.h    14 Sep 2014 17:39:06 -0000    1.57
> +++ ./sys/sys/bootblock.h    15 Sep 2014 02:50:00 -0000
> @@ -815,7 +815,7 @@
>      uint32_t    descBlock;    /* first block of driver */
>      uint16_t    descSize;    /* driver size in blocks */
>      uint16_t    descType;    /* system type */
> -} __packed;
> +};
> 
>  /*
>   *    system types; Apple reserves 0-15
> @@ -834,6 +834,10 @@
>      uint32_t    sbData;        /* (used internally by ROM) */
>      uint16_t    sbDrvrCount;    /* number of driver descriptors */
>      struct apple_drvr_descriptor sb_dd[APPLE_DRVR_MAP_MAX_DESCRIPTORS];
> +                    /* XXX: Don't access directly.
> +                     * Data abort occurs by an access to
> +                     * unaligned address on arm.
> +                     */
>      uint16_t    pad[3];
>  } __packed;

If the only problem is that some of the uint32_t are 16bit aligned
then use a 32bit type that has __attribute__((aligned(2)) applied to it.

Then the compiler can directlt access the uint16_t fields and need only
use two 16bit accesses for the 32bit ones.

That is already done for the mbr (which has a misaligned 32bit sector
number) and could be in the same file.
Also to get a 4-byte aligned 64bit quantity for amd64's i386 compat.

As mentioned later the memcpy() version won't work.

	David

-- 
David Laight: david%l8s.co.uk@localhost