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

[Jamie Lokier <jamie@shareable.org>]

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).

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?

-- Jamie