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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 23 Nov 2012, Ian Campbell wrote:
> 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.

You are right. Thinking twice about it, I would rather keep the code as
it was originally, for consistency with Linux.


> 
> > +   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".

Yep.

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