Re: arm: Only load TLS values when needed

[Jonathan Austin <jonathan.austin@arm.com>]

Hi André

(I've put RMK and LAKML back on Cc: they got dropped somewhere along the 
way)

On 14/08/13 22:21, André Hentschel wrote:
>>>>> [...]
>>>>>>> Now we've only got one instruction between the store and the load
>>>>>>> and risk stalling the pipeline...
>>>>>>>
>>>>>>> Dave M cautiously says "The ancient advice was that one instruction
>>>>>>> was enough" but this is very core dependent... I wonder if anyone
>>>>>>> has a good idea about whether this is an issue here...?
>>>>>>
>>>>>> We could use a ldrd at the top, that'd be nearly what we have right
>>>>>> now, don't we?
>>>>>
>>>>> Yea, that'd be good - as far as I can see from an 1136 TRM, the ldrd *may* be two cycles (depending on alignment of the words) but the ldr and ldrne will always be two cycles. Ahhh, the joys of modifying the fast path ;)
>>
>> Was expecting to see something that reflected this discussion,
>
> Ah ok, i misunderstood that, sry.
> Something like that?
>
> From: André Hentschel <nerv@dawncrow.de>
>
> This patch intents to reduce loading instructions when the resulting value is not used.
> It's a follow up on a4780adeefd042482f624f5e0d577bf9cdcbb760
>

You caught me just before running away for a week of holiday, but I've 
managed to squeeze in a bit of testing - hope this helps - I'll try to 
be clear about what I have/haven't tested so you can judge for yourself 
how much weight to give this.

What I've tested:

* Apply your code to 3.11-rc5
* Boot on Integrator CP (1176 r1p0) and run a full Debian Wheezy init

--> It works!

I also applied the following diff and rebuilt to check that we were 
really testing the V6 NOT V6K path here:

diff --git a/arch/arm/include/asm/tls.h b/arch/arm/include/asm/tls.h
index 31743f7..71dfe82 100644
--- a/arch/arm/include/asm/tls.h
+++ b/arch/arm/include/asm/tls.h
@@ -43,6 +43,7 @@
  #define switch_tls     switch_tls_v6
  #elif defined(CONFIG_CPU_32v6K)
  #define tls_emu                0
+#error
  #define has_tls_reg            1
  #define switch_tls     switch_tls_v6k
  #else

What I've *not* tested:
* As this CPU does not have the tls register, I haven't run any tests 
that try to read/write it, but we wouldn't expect that to work, right?
* Haven't tried a V7 platform, as I understand you've already done that.
* Haven't verified it still builds for v4 (which was the problem for 
ldrd in the past and now we've put it back, so that's important)
* Performance impact

I think it'd be good if you can verify the bottom two points - 
especially the ldrd one: perhaps Russell can tell you which platform it 
broke on before?

So, that's a
Tested-by: Jonathan Austin <jonathan.austin@arm.com>

If you want it :)
Hope that helps,

Jonny

> Signed-off-by: André Hentschel <nerv@dawncrow.de>
>
> ---
> This patch is against 28fbc8b6a29c849a3f03a6b05010d4b584055665
>
> diff --git a/arch/arm/include/asm/tls.h b/arch/arm/include/asm/tls.h
> index 83259b8..31743f7 100644
> --- a/arch/arm/include/asm/tls.h
> +++ b/arch/arm/include/asm/tls.h
> @@ -3,29 +3,31 @@
>
>   #ifdef __ASSEMBLY__
>   #include <asm/asm-offsets.h>
> -	.macro switch_tls_none, base, tp, tpuser, tmp1, tmp2
> +	.macro switch_tls_none, prev, next, tp, tpuser, tmp1, tmp2
>   	.endm
>
> -	.macro switch_tls_v6k, base, tp, tpuser, tmp1, tmp2
> +	.macro switch_tls_v6k, prev, next, tp, tpuser, tmp1, tmp2
> +	ldrd	\tp, \tpuser, [\next, #TI_TP_VALUE]	@ get the next TLS and user r/w register
>   	mrc	p15, 0, \tmp2, c13, c0, 2	@ get the user r/w register
>   	mcr	p15, 0, \tp, c13, c0, 3		@ set TLS register
>   	mcr	p15, 0, \tpuser, c13, c0, 2	@ and the user r/w register
> -	str	\tmp2, [\base, #TI_TP_VALUE + 4] @ save it
> +	str	\tmp2, [\prev, #TI_TP_VALUE + 4] @ save it
>   	.endm
>
> -	.macro switch_tls_v6, base, tp, tpuser, tmp1, tmp2
> +	.macro switch_tls_v6, prev, next, tp, tpuser, tmp1, tmp2
> +	ldrd	\tp, \tpuser, [\next, #TI_TP_VALUE]
>   	ldr	\tmp1, =elf_hwcap
>   	ldr	\tmp1, [\tmp1, #0]
>   	mov	\tmp2, #0xffff0fff
>   	tst	\tmp1, #HWCAP_TLS		@ hardware TLS available?
>   	streq	\tp, [\tmp2, #-15]		@ set TLS value at 0xffff0ff0
> -	mrcne	p15, 0, \tmp2, c13, c0, 2	@ get the user r/w register
> +	mrcne	p15, 0, \tmp2, c13, c0, 2	@ get the previous user r/w register
>   	mcrne	p15, 0, \tp, c13, c0, 3		@ yes, set TLS register
>   	mcrne	p15, 0, \tpuser, c13, c0, 2	@ set user r/w register
> -	strne	\tmp2, [\base, #TI_TP_VALUE + 4] @ save it
> +	strne	\tmp2, [\prev, #TI_TP_VALUE + 4] @ save it
>   	.endm
>
> -	.macro switch_tls_software, base, tp, tpuser, tmp1, tmp2
> +	.macro switch_tls_software, prev, next, tp, tpuser, tmp1, tmp2
>   	mov	\tmp1, #0xffff0fff
>   	str	\tp, [\tmp1, #-15]		@ set TLS value at 0xffff0ff0
>   	.endm
> diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> index d40d0ef..11112de 100644
> --- a/arch/arm/kernel/entry-armv.S
> +++ b/arch/arm/kernel/entry-armv.S
> @@ -689,12 +689,10 @@ ENTRY(__switch_to)
>    THUMB(	stmia	ip!, {r4 - sl, fp}	   )	@ Store most regs on stack
>    THUMB(	str	sp, [ip], #4		   )
>    THUMB(	str	lr, [ip], #4		   )
> -	ldr	r4, [r2, #TI_TP_VALUE]
> -	ldr	r5, [r2, #TI_TP_VALUE + 4]
>   #ifdef CONFIG_CPU_USE_DOMAINS
>   	ldr	r6, [r2, #TI_CPU_DOMAIN]
>   #endif
> -	switch_tls r1, r4, r5, r3, r7
> +	switch_tls r1, r2, r4, r5, r3, r7
>   #if defined(CONFIG_CC_STACKPROTECTOR) && !defined(CONFIG_SMP)
>   	ldr	r7, [r2, #TI_TASK]
>   	ldr	r8, =__stack_chk_guard
>
>
>