linux-kernel - Re: [PATCH v2 1/3] perf tools: Use offset instead of dwarfnum in register table.

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Message-ID: <20170203130026.GF11585@arm.com>
Date:   Fri, 3 Feb 2017 13:00:26 +0000
From:   Will Deacon <will.deacon@....com>
To:     He Kuang <hekuang@...wei.com>
Cc:     peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, mhiramat@...nel.org,
        jolsa@...hat.com, wangnan0@...wei.com, bintian.wang@...wei.com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/3] perf tools: Use offset instead of dwarfnum in
 register table.

On Fri, Feb 03, 2017 at 11:06:05AM +0000, He Kuang wrote:
> This patch changes the 'dwarfnum' to 'offset' in register table, so
> the index of array becomes the dwarfnum (the index of each register
> defined by DWARF) and the "offset" member means the byte-offset of the
> register in (user_)pt_regs. This change makes the code consistent with
> x86.
> 
> Acked-by: Masami Hiramatsu <mhiramat@...nel.org>
> Signed-off-by: He Kuang <hekuang@...wei.com>
> ---
>  tools/perf/arch/arm64/util/dwarf-regs.c | 107 ++++++++++++++++----------------
>  1 file changed, 52 insertions(+), 55 deletions(-)

Thanks for splitting this up. Comment below.

> diff --git a/tools/perf/arch/arm64/util/dwarf-regs.c b/tools/perf/arch/arm64/util/dwarf-regs.c
> index d49efeb..090f36b 100644
> --- a/tools/perf/arch/arm64/util/dwarf-regs.c
> +++ b/tools/perf/arch/arm64/util/dwarf-regs.c
> @@ -9,72 +9,69 @@
>   */
>  
>  #include <stddef.h>
> +#include <linux/ptrace.h> /* for struct user_pt_regs */
>  #include <dwarf-regs.h>
>  
> -struct pt_regs_dwarfnum {
> +struct pt_regs_offset {
>  	const char *name;
> -	unsigned int dwarfnum;
> +	int offset;
>  };
>  
> -#define STR(s) #s
> -#define REG_DWARFNUM_NAME(r, num) {.name = r, .dwarfnum = num}
> -#define GPR_DWARFNUM_NAME(num) \
> -	{.name = STR(%x##num), .dwarfnum = num}
> -#define REG_DWARFNUM_END {.name = NULL, .dwarfnum = 0}
> -
>  /*
>   * Reference:
>   * http://infocenter.arm.com/help/topic/com.arm.doc.ihi0057b/IHI0057B_aadwarf64.pdf
>   */
> -static const struct pt_regs_dwarfnum regdwarfnum_table[] = {
> -	GPR_DWARFNUM_NAME(0),
> -	GPR_DWARFNUM_NAME(1),
> -	GPR_DWARFNUM_NAME(2),
> -	GPR_DWARFNUM_NAME(3),
> -	GPR_DWARFNUM_NAME(4),
> -	GPR_DWARFNUM_NAME(5),
> -	GPR_DWARFNUM_NAME(6),
> -	GPR_DWARFNUM_NAME(7),
> -	GPR_DWARFNUM_NAME(8),
> -	GPR_DWARFNUM_NAME(9),
> -	GPR_DWARFNUM_NAME(10),
> -	GPR_DWARFNUM_NAME(11),
> -	GPR_DWARFNUM_NAME(12),
> -	GPR_DWARFNUM_NAME(13),
> -	GPR_DWARFNUM_NAME(14),
> -	GPR_DWARFNUM_NAME(15),
> -	GPR_DWARFNUM_NAME(16),
> -	GPR_DWARFNUM_NAME(17),
> -	GPR_DWARFNUM_NAME(18),
> -	GPR_DWARFNUM_NAME(19),
> -	GPR_DWARFNUM_NAME(20),
> -	GPR_DWARFNUM_NAME(21),
> -	GPR_DWARFNUM_NAME(22),
> -	GPR_DWARFNUM_NAME(23),
> -	GPR_DWARFNUM_NAME(24),
> -	GPR_DWARFNUM_NAME(25),
> -	GPR_DWARFNUM_NAME(26),
> -	GPR_DWARFNUM_NAME(27),
> -	GPR_DWARFNUM_NAME(28),
> -	GPR_DWARFNUM_NAME(29),
> -	REG_DWARFNUM_NAME("%lr", 30),
> -	REG_DWARFNUM_NAME("%sp", 31),
> -	REG_DWARFNUM_END,
> -};
> +#define REG_OFFSET_NAME(r, num) {.name = "%" #r,			\
> +			.offset = offsetof(struct user_pt_regs, regs[num])}

Whilst this works in practice, this is undefined behaviour for "sp", since
you'll go off the end of the regs array.

I still think you're better off sticking with the dwarfnum, then just having
a dwarfnum2offset macro that multiplies by the size of a register.

Will