linux-kernel - Re: [PATCH] recordmcount: arm64: replace the ignored mcount call into nop

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Date:	Wed, 28 Oct 2015 04:16:19 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Li Bin <huawei.libin@...wei.com>
Cc:	<catalin.marinas@....com>, <will.deacon@....com>,
	<takahiro.akashi@...aro.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <guohanjun@...wei.com>,
	<dingtianhong@...wei.com>, <zhouchengming1@...wei.com>
Subject: Re: [PATCH] recordmcount: arm64: replace the ignored mcount call
 into nop

On Wed, 28 Oct 2015 16:04:05 +0800
Li Bin <huawei.libin@...wei.com> wrote:

> By now, the recordmcount only records the function that in
> following sections:
> .text/.ref.text/.sched.text/.spinlock.text/.irqentry.text/
> .kprobes.text/.text.unlikely
> 
> For the function that not in these sections, the call mcount
> will be in place and not be replaced when kernel boot up. And
> it will bring performance overhead, such as do_mem_abort (in
> .exception.text section). This patch make the call mcount to
> nop for this case in recordmcount.

I can take this in my tree with an Ack from the ARM64 maintainers.

-- Steve

> 
> Signed-off-by: Li Bin <huawei.libin@...wei.com>
> ---
>  scripts/recordmcount.c | 23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c
> index 3d1984e..f697226 100644
> --- a/scripts/recordmcount.c
> +++ b/scripts/recordmcount.c
> @@ -42,6 +42,7 @@
>  
>  #ifndef EM_AARCH64
>  #define EM_AARCH64	183
> +#define R_AARCH64_NONE		0
>  #define R_AARCH64_ABS64	257
>  #endif
>  
> @@ -160,6 +161,21 @@ static int make_nop_x86(void *map, size_t const offset)
>  	return 0;
>  }
>  
> +static unsigned char ideal_nop4_arm64[4] = {0x1f, 0x20, 0x03, 0xd5};
> +static int make_nop_arm64(void *map, size_t const offset)
> +{
> +	uint32_t *ptr;
> +
> +	ptr = map + offset;
> +	if (*ptr != 0x94000000)
> +		return -1;
> +
> +	/* Convert to nop */
> +	ulseek(fd_map, offset, SEEK_SET);
> +	uwrite(fd_map, ideal_nop, 4);
> +	return 0;
> +}
> +
>  /*
>   * Get the whole file as a programming convenience in order to avoid
>   * malloc+lseek+read+free of many pieces.  If successful, then mmap
> @@ -353,7 +369,12 @@ do_file(char const *const fname)
>  			 altmcount = "__gnu_mcount_nc";
>  			 break;
>  	case EM_AARCH64:
> -			 reltype = R_AARCH64_ABS64; gpfx = '_'; break;
> +			reltype = R_AARCH64_ABS64;
> +			make_nop = make_nop_arm64;
> +			rel_type_nop = R_AARCH64_NONE;
> +			ideal_nop = ideal_nop4_arm64;
> +			gpfx = '_';
> +			break;
>  	case EM_IA_64:	 reltype = R_IA64_IMM64;   gpfx = '_'; break;
>  	case EM_METAG:	 reltype = R_METAG_ADDR32;
>  			 altmcount = "_mcount_wrapper";

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