Re: [Xen-devel] [PATCH v2] xen/arm: build as zImage

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2012-11-23 at 18:04 +0000, Stefano Stabellini wrote:
> The zImage format is extremely simple: it only consists of a magic
> number and 2 addresses in a specific position (see
> http://www.simtec.co.uk/products/SWLINUX/files/booting_article.html#d0e309).
> 
> Some bootloaders expect a zImage; considering that it doesn't cost us
> much to build Xen compatible with the format, make it so.
> 
> Changes in v2:
> - code style;
> - pass 0 a start address.
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> 
> diff --git a/xen/arch/arm/head.S b/xen/arch/arm/head.S
> index 25c4cfe..347ddcf 100644
> --- a/xen/arch/arm/head.S
> +++ b/xen/arch/arm/head.S
> @@ -22,6 +22,8 @@
>  #include <asm/processor-ca15.h>
>  #include <asm/asm_defns.h>
>  
> +#define ZIMAGE_MAGIC_NUMBER 0x016f2818
> +
>  #define PT_PT  0xe7f /* nG=1, AF=1, SH=10, AP=01, NS=1, ATTR=111, T=1, P=1 */
>  #define PT_MEM 0xe7d /* nG=1, AF=1, SH=10, AP=01, NS=1, ATTR=111, T=0, P=1 */
>  #define PT_DEV 0xe71 /* nG=1, AF=1, SH=10, AP=01, NS=1, ATTR=100, T=0, P=1 */
> @@ -52,6 +54,18 @@
>        * or the initial pagetable code below will need adjustment. */
>       .global start
>  start:
> +
> +     /* zImage magic header, see:
> +      * 
> http://www.simtec.co.uk/products/SWLINUX/files/booting_article.html#d0e309
> +      */
> +     .skip 0x20

This was better as the explicit repeated nops, as it stands it's not at
all clear that you can start executing at start. It might even rely on
the specific assemblers behaviour?

Also this wasn't what Tim suggested, which was branch then skip, not
skip then branch. I think the nops are better for consistency with Linux
though.

> +     b     past_zImage
> +     .word ZIMAGE_MAGIC_NUMBER    /* Magic numbers to help the loader */
> +     .word 0x00000000             /* absolute load/run zImage address or
> +                                   * 0 for PiC */
> +     .word _end                   /* zImage end address */

This needs to be "_end - _start".

Ian.


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