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    SubjectRe: [PATCH 08/12] powerpc/ftrace: Rework ftrace_caller()
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    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 <mpe@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 <bsingharora@gmail.com>

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