Subject: Re: [RFC PATCH 1/2]powerpc: foundation code to handle CR5 for local_t

From: Scott Wood <[email-masked]>
Date: Mon, 1 Dec 2014 20:04:50 -0600

On Thu, 2014-11-27 at 17:48 +0530, Madhavan Srinivasan wrote:
> - I really appreciate feedback on the patchset.
> - Kindly comment if I should try with any other benchmark or
>     workload to check the numbers.
> - Also, kindly recommand any know stress test for CR
> 
>  Makefile                                 |   6 ++
>  arch/powerpc/include/asm/exception-64s.h |  21 +++++-
>  arch/powerpc/kernel/entry_64.S           | 106 ++++++++++++++++++++++++++++++-
>  arch/powerpc/kernel/exceptions-64s.S     |   2 +-
>  arch/powerpc/kernel/head_64.S            |   8 +++
>  5 files changed, 138 insertions(+), 5 deletions(-)

Patch 2/2 enables this for all PPC64, not just book3s -- so please don't
forget about the book3e exception paths (also MSR[GS] for KVM, but
aren't most if not all the places you're checking for HV mode after KVM
would have taken control?  Or am I missing something about how book3s
KVM works?).

Or, if you don't want to do that, change patch 2/2 to be book3s only and
ifdef-protect the changes to common exception code.

> @@ -224,8 +243,26 @@ syscall_exit:
>  BEGIN_FTR_SECTION
>  	stdcx.	r0,0,r1			/* to clear the reservation */
>  END_FTR_SECTION_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
> +BEGIN_FTR_SECTION
> +	lis	r4,4096
> +	rldicr	r4,r4,32,31
> +	mr	r6,r4
> +	ori	r4,r4,16384
> +	and	r4,r8,r4
> +	cmpd	cr3,r6,r4
> +	beq	cr3,65f
> +	mtcr	r5
> +FTR_SECTION_ELSE
>  	andi.	r6,r8,MSR_PR
> -	ld	r4,_LINK(r1)
> +	beq	65f
> +	mtcr	r5
> +	nop
> +	nop
> +	nop
> +	nop
> +	nop
> +ALT_FTR_SECTION_END_IFSET(CPU_FTR_HVMODE)
> +65:	ld	r4,_LINK(r1)
>  
>  	beq-	1f
>  	ACCOUNT_CPU_USER_EXIT(r11, r12)
> @@ -234,7 +271,11 @@ END_FTR_SECTION_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
>  1:	ld	r2,GPR2(r1)
>  	ld	r1,GPR1(r1)
>  	mtlr	r4
> +#ifdef	CONFIG_PPC64
> +	mtcrf	0xFB,r5
> +#else
>  	mtcr	r5
> +#endif

mtcrf with more than one CRn being updated is expensive on Freescale
chips (and this isn't a book3s-only code path).  Why do you need to do
it twice?  I don't see where either r5 or cr5 are messed with between
the two places...

-Scott