On 25/02/16 01:28, Michael Ellerman wrote:
> The main change is to just use paca->kernel_toc, rather than a branch to
> +4 and mflr etc. That makes the code simpler and should also perform
> better.
>
> There was also a sequence after ftrace_call() where we load from
> pt_regs->nip, move to LR, then a few instructions later load from LRSAVE
> and move to LR. Instead I think we want to put pt_regs->nip into CTR and
> branch to it later.
>
> We also rework some of the SPR loads to hopefully speed them up a bit.
> Also comment the asm much more, to hopefully make it clearer.
>
> Signed-off-by: Michael Ellerman <m...@ellerman.id.au>
> ---
>  arch/powerpc/kernel/entry_64.S | 94 
> ++++++++++++++++++++++++++++--------------
>  1 file changed, 62 insertions(+), 32 deletions(-)
>
> Squash.
>
> diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
> index 149b659a25d9..f347f50024b8 100644
> --- a/arch/powerpc/kernel/entry_64.S
> +++ b/arch/powerpc/kernel/entry_64.S
> @@ -1171,65 +1171,98 @@ _GLOBAL(ftrace_graph_stub)
>       mtlr    r0
>       addi    r1, r1, 112
>  #else
> +/*
> + *
> + * ftrace_caller() is the function that replaces _mcount() when ftrace is
> + * active.
> + *
> + * We arrive here after a function A calls function B, and we are the trace
> + * function for B. When we enter r1 points to A's stack frame, B has not yet
> + * had a chance to allocate one yet.
> + *
> + * Additionally r2 may point either to the TOC for A, or B, depending on
> + * whether B did a TOC setup sequence before calling us.
> + *
> + * On entry the LR points back to the _mcount() call site, and r0 holds the
> + * saved LR as it was on entry to B, ie. the original return address at the
> + * call site in A.
> + *
> + * Our job is to save the register state into a struct pt_regs (on the stack)
> + * and then arrange for the ftrace function to be called.
> + */
>  _GLOBAL(ftrace_caller)
> +     /* Save the original return address in A's stack frame */
>       std     r0,LRSAVE(r1)
> -#if defined(_CALL_ELF) && _CALL_ELF == 2
> -     mflr    r0
> -     bl      2f
> -2:   mflr    r12
> -     mtlr    r0
> -     mr      r0,r2   /* save callee's TOC */
> -     addis   r2,r12,(.TOC.-ftrace_caller-12)@ha
> -     addi    r2,r2,(.TOC.-ftrace_caller-12)@l
> -#else
> -     mr      r0,r2
> -#endif
> -     ld      r12,LRSAVE(r1)  /* get caller's address */
>  
> +     /* Create our stack frame + pt_regs */
>       stdu    r1,-SWITCH_FRAME_SIZE(r1)
>  
> -     std     r12, _LINK(r1)
> +     /* Save all gprs to pt_regs */
>       SAVE_8GPRS(0,r1)
> -     std     r0, 24(r1)      /* save TOC */
>       SAVE_8GPRS(8,r1)
>       SAVE_8GPRS(16,r1)
>       SAVE_8GPRS(24,r1)
>  
> +     /* Load special regs for save below */
> +     mfmsr   r8
> +     mfctr   r9
> +     mfxer   r10
> +     mfcr    r11
> +
> +     /* Get the _mcount() call site out of LR */
> +     mflr    r7
> +     /* Save it as pt_regs->nip & pt_regs->link */
> +     std     r7, _NIP(r1)
> +     std     r7, _LINK(r1)
> +
> +     /* Save callee's TOC in the ABI compliant location */
> +     std     r2, 24(r1)
> +     ld      r2,PACATOC(r13) /* get kernel TOC in r2 */
> +
>       addis   r3,r2,function_trace_op@toc@ha
>       addi    r3,r3,function_trace_op@toc@l
>       ld      r5,0(r3)
>  
> -     mflr    r3
> -     std     r3, _NIP(r1)
> -     std     r3, 16(r1)
> -     subi    r3, r3, MCOUNT_INSN_SIZE
> -     mfmsr   r4
> -     std     r4, _MSR(r1)
> -     mfctr   r4
> -     std     r4, _CTR(r1)
> -     mfxer   r4
> -     std     r4, _XER(r1)
> -     mr      r4, r12
> +     /* Calculate ip from nip-4 into r3 for call below */
> +     subi    r3, r7, MCOUNT_INSN_SIZE
> +
> +     /* Put the original return address in r4 as parent_ip */
> +     mr      r4, r0
> +
> +     /* Save special regs */
> +     std     r8, _MSR(r1)
> +     std     r9, _CTR(r1)
> +     std     r10, _XER(r1)
> +     std     r11, _CCR(r1)
> +
> +     /* Load &pt_regs in r6 for call below */
>       addi    r6, r1 ,STACK_FRAME_OVERHEAD
>  
> +     /* ftrace_call(r3, r4, r5, r6) */
>  .globl ftrace_call
>  ftrace_call:
>       bl      ftrace_stub
>       nop
>  
> +     /* Load ctr with the possibly modified NIP */
>       ld      r3, _NIP(r1)
> -     mtlr    r3
> +     mtctr   r3
>  
> +     /* Restore gprs */
>       REST_8GPRS(0,r1)
>       REST_8GPRS(8,r1)
>       REST_8GPRS(16,r1)
>       REST_8GPRS(24,r1)
>  
> +     /* Restore callee's TOC */
> +     ld      r2, 24(r1)
> +
> +     /* Pop our stack frame */
>       addi r1, r1, SWITCH_FRAME_SIZE
>  
> -     ld      r12, LRSAVE(r1)  /* get caller's address */
> -     mtlr    r12
> -     mr      r2,r0           /* restore callee's TOC */
> +     /* Restore original LR for return to B */
> +     ld      r0, LRSAVE(r1)
> +     mtlr    r0
>  
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>       stdu    r1, -112(r1)
> @@ -1240,9 +1273,6 @@ _GLOBAL(ftrace_graph_stub)
>       addi    r1, r1, 112
>  #endif
>  
> -     mflr    r0              /* move this LR to CTR */
> -     mtctr   r0
> -
>       ld      r0,LRSAVE(r1)   /* restore callee's lr at _mcount site */
>       mtlr    r0
>       bctr                    /* jump after _mcount site */

Reviewed-by: Balbir Singh <bsinghar...@gmail.com>
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