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

[Tim Deegan <tim@xen.org>]

At 12:11 +0000 on 27 Nov (1354018273), Stefano Stabellini wrote:
> 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@eu.citrix.com>
> > > 
> > > 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.

Does consistency with linux buy us anything (beyond untidy code)?
I very much prefer the other way.  And if we do need to fill with nops,
why 7 repeated and one standalone?

Tim.