Re: [PATCH 1/2] arm: Replace CONFIG_HAS_TLS_REG with HWCAP_TLS and check for it on V6

[Tony Lindgren <tony@atomide.com>]

* Jamie Lokier <jamie@shareable.org> [100622 19:54]:
> Tony Lindgren wrote:
> >  __kuser_get_tls:				@ 0xffff0fe0
> > -
> > -#if !defined(CONFIG_HAS_TLS_REG) && !defined(CONFIG_TLS_REG_EMUL)
> > -	ldr	r0, [pc, #(16 - 8)]		@ TLS stored at 0xffff0ff0
> > -#else
> > -	mrc	p15, 0, r0, c13, c0, 3		@ read TLS register
> > -#endif
> > +	ldr     r0, [pc, #(20 - 8)]		@ software TLS set in 0xffff0ff4?
> > +	cmp	r0, #0				@ hardware TLS if flag not set
> > +	mrceq	p15, 0, r0, c13, c0, 3		@ read hardware TLS register
> > +	ldrne	r0, [pc, #(12 - 8)]		@ software TLS val at 0xffff0ff8
> >  	usr_ret	lr
> > -
> > -	.rep	5
> > -	.word	0			@ pad up to __kuser_helper_version
> > -	.endr
> > +	.word	0				@ non-zero for software TLS
> > +	.word	0				@ software TLS value
> 
> It'd be nice not to waste instructions checking for HWCAP_TLS on archs
> which definitely don't have it.  I guess it doesn't matter elsewhere;
> I'd expect this to be a warm path for some programs making extensive
> use of TLS (I haven't measured though).

OK, but let's try to figure out a way that does not add more ifdef else
code as that makes it harder to build support for multiple ARM cores.
 
> As it's only a single instruction, and the code is in a writable page
> already (copied at init), how about just patching the instruction
> when ELF_HWCAP is set?

Yeah that can be done for __kuser_get_tls if it's always writable.
But __switch_to is trickier because of the CONFIG_MMU ifdefs there.

What if we have optional __switch_to and __kuser_get_tls implementations
in the mm/proc-*.S files that get copied over the current locations
if implemented?

Regards,

Tony