[RFC PATCH 5/5] ARM: entry: Fix Thumb-2 undef handling for multi-CPU kernels

[Dave Martin]

On Thu, Sep 01, 2011 at 04:52:12PM +0200, Arnd Bergmann wrote:
> On Wednesday 10 August 2011, Dave Martin wrote:
> > When v6 and >=v7 boards are supported in the same kernel, the
> > __und_usr code currently makes a build-time assumption that Thumb-2
> > instructions occurring in userspace don't need to be supported.
> > Strictly speaking this is incorrect.
> > 
> > This patch fixes the above case by doing a run-time check on the
> > CPU architecture in these cases.  This only affects kernels which
> > support v6 and >=v7 CPUs together: plain v6 and plain v7 kernels
> > are unaffected.
> 
> I think this patch broke random configurations (!THUMB2 && v7) for me:
> 
> > @@ -439,7 +440,27 @@ __und_usr:
> >  #endif
> >  	beq	call_fpe
> >  	@ Thumb instruction
> > -#if __LINUX_ARM_ARCH__ >= 7
> > +#if CONFIG_ARM_THUMB && __LINUX_ARM_ARCH__ >= 6 && CONFIG_CPU_V7
> > +/*
> > + * Thumb-2 instruction handling.  Note that because pre-v6 and >= v6 platforms
> > + * can never be supported in a single kernel, this code is not applicable at
> > + * all when __LINUX_ARM_ARCH__ < 6.  This allows simplifying assumptions to be
> > + * made about .arch directives.
> > + */
> > +#if __LINUX_ARM_ARCH__ < 7
> > +/* If the target CPU may not be Thumb-2-capable, a run-time check is needed: */
> > +#define NEED_CPU_ARCHITECTURE
> > +	ldr	r5, .LCcpu_architecture
> > +	ldr	r5, [r5]
> > +	cmp	r5, #CPU_ARCH_ARMv7
> > +	blo	__und_usr_unknown
> > +/*
> > + * The following code won't get run unless the running CPU really is v7, so
> > + * coding round the lack of ldrht on older arches is pointless.  Temporarily
> > + * override the assembler target arch with the minimum required instead:
> > + */
> > +	.arch	armv6t2
> > +#endif
> >  2:
> >   ARM(	ldrht	r5, [r4], #2	)
> 
> 
> This fixes it, please fold into the patch or apply on top.
> 
> 	Arnd
> 
> 8<---
> ARM: entry: refix Fix Thumb-2 undef handling for multi-CPU kernels
> 
> In some configurations labels become obsolete, so don't generate fixups for them.
> 
> Signed-off-by: Arnd Bergmann <arnd at arndb.de>
> 
> diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> index 9ad50c4..b145f16 100644
> --- a/arch/arm/kernel/entry-armv.S
> +++ b/arch/arm/kernel/entry-armv.S
> @@ -497,7 +497,7 @@ ENDPROC(__und_usr)
>  	.popsection
>  	.pushsection __ex_table,"a"
>  	.long	1b, 4b
> -#if __LINUX_ARM_ARCH__ >= 7
> +#if CONFIG_ARM_THUMB && __LINUX_ARM_ARCH__ >= 6 && CONFIG_CPU_V7
>  	.long	2b, 4b
>  	.long	3b, 4b
>  #endif

This patch looks correct.

We could tidy things up so this is easier to maintain, by avoiding the
fixups being declared in a separate place from the labels they
reference, but I'm not sure if it's worth it just for this case.

Russell, would you prefer to fold this in with the previous patch, or
apply it on top?

Cheers
---Dave