Subject: Re: [RFC PATCH v3 02/11] powerpc/ftrace: Unify 32-bit and 64-bit ftrace entry code

From: "Nicholas Piggin" <[email-masked]>
Date: Mon, 01 Jul 2024 18:57:12 +1000

On Fri Jun 21, 2024 at 4:54 AM AEST, Naveen N Rao wrote:
> On 32-bit powerpc, gcc generates a three instruction sequence for
> function profiling:
> 	mflr	r0
> 	stw	r0, 4(r1)
> 	bl	_mcount
>
> On kernel boot, the call to _mcount() is nop-ed out, to be patched back
> in when ftrace is actually enabled. The 'stw' instruction therefore is
> not necessary unless ftrace is enabled. Nop it out during ftrace init.
>
> When ftrace is enabled, we want the 'stw' so that stack unwinding works
> properly. Perform the same within the ftrace handler, similar to 64-bit
> powerpc.
>
> For 64-bit powerpc, early versions of gcc used to emit a three
> instruction sequence for function profiling (with -mprofile-kernel) with
> a 'std' instruction to mimic the 'stw' above. Address that scenario also
> by nop-ing out the 'std' instruction during ftrace init.

Cool! Could 32-bit use the 2-insn sequence as well if it had
-mprofile-kernel, out of curiosity?

>
> Signed-off-by: Naveen N Rao <[email-masked]>
> ---
>  arch/powerpc/kernel/trace/ftrace.c       | 6 ++++--
>  arch/powerpc/kernel/trace/ftrace_entry.S | 4 ++--
>  2 files changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/arch/powerpc/kernel/trace/ftrace.c b/arch/powerpc/kernel/trace/ftrace.c
> index d8d6b4fd9a14..463bd7531dc8 100644
> --- a/arch/powerpc/kernel/trace/ftrace.c
> +++ b/arch/powerpc/kernel/trace/ftrace.c
> @@ -241,13 +241,15 @@ int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
>  		/* Expected sequence: 'mflr r0', 'stw r0,4(r1)', 'bl _mcount' */
>  		ret = ftrace_validate_inst(ip - 8, ppc_inst(PPC_RAW_MFLR(_R0)));
>  		if (!ret)
> -			ret = ftrace_validate_inst(ip - 4, ppc_inst(PPC_RAW_STW(_R0, _R1, 4)));
> +			ret = ftrace_modify_code(ip - 4, ppc_inst(PPC_RAW_STW(_R0, _R1, 4)),
> +						 ppc_inst(PPC_RAW_NOP()));
>  	} else if (IS_ENABLED(CONFIG_MPROFILE_KERNEL)) {
>  		/* Expected sequence: 'mflr r0', ['std r0,16(r1)'], 'bl _mcount' */
>  		ret = ftrace_read_inst(ip - 4, &old);
>  		if (!ret && !ppc_inst_equal(old, ppc_inst(PPC_RAW_MFLR(_R0)))) {
>  			ret = ftrace_validate_inst(ip - 8, ppc_inst(PPC_RAW_MFLR(_R0)));
> -			ret |= ftrace_validate_inst(ip - 4, ppc_inst(PPC_RAW_STD(_R0, _R1, 16)));
> +			ret |= ftrace_modify_code(ip - 4, ppc_inst(PPC_RAW_STD(_R0, _R1, 16)),
> +						  ppc_inst(PPC_RAW_NOP()));

So this is the old style path... Should you check the mflr validate
result first? Also do you know what GCC version, roughly? Maybe we
could have a comment here and eventually deprecate it.

You could split this change into its own patch.

>  		}
>  	} else {
>  		return -EINVAL;
> diff --git a/arch/powerpc/kernel/trace/ftrace_entry.S b/arch/powerpc/kernel/trace/ftrace_entry.S
> index 76dbe9fd2c0f..244a1c7bb1e8 100644
> --- a/arch/powerpc/kernel/trace/ftrace_entry.S
> +++ b/arch/powerpc/kernel/trace/ftrace_entry.S
> @@ -33,6 +33,8 @@
>   * and then arrange for the ftrace function to be called.
>   */
>  .macro	ftrace_regs_entry allregs
> +	/* Save the original return address in A's stack frame */
> +	PPC_STL		r0, LRSAVE(r1)
>  	/* Create a minimal stack frame for representing B */
>  	PPC_STLU	r1, -STACK_FRAME_MIN_SIZE(r1)
>  
> @@ -44,8 +46,6 @@
>  	SAVE_GPRS(3, 10, r1)
>  
>  #ifdef CONFIG_PPC64
> -	/* Save the original return address in A's stack frame */
> -	std	r0, LRSAVE+SWITCH_FRAME_SIZE+STACK_FRAME_MIN_SIZE(r1)
>  	/* Ok to continue? */
>  	lbz	r3, PACA_FTRACE_ENABLED(r13)
>  	cmpdi	r3, 0

That seems right to me.

Reviewed-by: Nicholas Piggin <[email-masked]>