linux-kernel - Re: [PATCH 04/12] powerpc/ftrace: Prepare for -mprofile-kernel

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Date:	Thu, 25 Feb 2016 14:52:05 +0100
From:	Torsten Duwe <duwe@....de>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	linuxppc-dev@...abs.org, bsingharora@...il.com,
	linux-kernel@...r.kernel.org, rostedt@...dmis.org,
	kamalesh@...ux.vnet.ibm.com, pmladek@...e.com, jeyu@...hat.com,
	jkosina@...e.cz, live-patching@...r.kernel.org, mbenes@...e.cz
Subject: Re: [PATCH 04/12] powerpc/ftrace: Prepare for -mprofile-kernel

On Thu, Feb 25, 2016 at 01:28:27AM +1100, Michael Ellerman wrote:
> @@ -450,17 +448,44 @@ static unsigned long stub_for_addr(const Elf64_Shdr *sechdrs,
>  	return (unsigned long)&stubs[i];
>  }
>  
> +#ifdef CC_USING_MPROFILE_KERNEL
> +static int is_early_mcount_callsite(u32 *instruction)
> +{
> +	/* -mprofile-kernel sequence starting with
> +	 * mflr r0 and maybe std r0, LRSAVE(r1).
> +	 */
> +	if ((instruction[-3] == PPC_INST_MFLR &&
> +	     instruction[-2] == PPC_INST_STD_LR) ||
> +	    instruction[-2] == PPC_INST_MFLR) {
> +		/* Nothing to be done here, it's an _mcount
> +		 * call location and r2 will have to be
> +		 * restored in the _mcount function.
> +		 */
> +		return 1;
> +	}
> +	return 0;
> +}
> +#else

*You* said this might page fault :)

Did we agree yet whether we insist on a streamlined compiler?
(GCC commit e95d0248dace required)?

If not:
if (instruction[-2] == PPC_INST_STD_LR)
  {
    if (instruction[-3] == PPC_INST_MFLR)
      return 1;
  }
else if (instruction[-2] == PPC_INST_MFLR)
    return 1;
return 0;

leaves less freedom for the compiler to "optimise".

Signed-off-by: Torsten Duwe <duwe@...e.de>

	Torsten