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Date: Wed, 7 Mar 2018 12:45:53 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>, Michael Ellerman
 <mpe@ellerman.id.au>, Paul Mackerras <paulus@ozlabs.org>, Anton Blanchard
 <anton@samba.org>, Nicholas Piggin <npiggin@gmail.com>,
 linuxppc-dev@lists.ozlabs.org
Subject: Re: [RFC PATCH 1/1] powerpc/ftrace: Exclude real mode code from
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On Wed,  7 Mar 2018 22:16:19 +0530
"Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:

> We can't take a trap in most parts of real mode code. Instead of adding
> the 'notrace' annotation to all C functions that can be invoked from
> real mode, detect that we are in real mode on ftrace entry and return
> back.
> 
> Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
> ---
> This RFC only handles -mprofile-kernel to demonstrate the approach being 
> considered. We will need to handle other ftrace entry if we decide to 
> continue down this path.

I do prefer this trade off.


> diff --git a/arch/powerpc/kernel/trace/ftrace_64_mprofile.S b/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
> index 3f3e81852422..ecc0e8e38ead 100644
> --- a/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
> +++ b/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
> @@ -56,6 +56,21 @@ _GLOBAL(ftrace_caller)
>  
>  	/* Load special regs for save below */
>  	mfmsr   r8
> +
> +	/* Only proceed if we are not in real mode and can take interrupts */
> +	andi.	r9, r8, MSR_IR|MSR_DR|MSR_RI
> +	cmpdi	r9, MSR_IR|MSR_DR|MSR_RI
> +	beq	1f

OK, I assume this check and branch is negligible compared to the mfmsr
call?

-- Steve


> +	mflr	r8
> +	mtctr	r8
> +	REST_GPR(9, r1)
> +	REST_GPR(8, r1)
> +	addi	r1, r1, SWITCH_FRAME_SIZE
> +	ld	r0, LRSAVE(r1)
> +	mtlr	r0
> +	bctr
> +
> +1:
>  	mfctr   r9
>  	mfxer   r10
>  	mfcr	r11