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

From: Will Deacon <will.deacon@arm.com>
To: Hekuang <hekuang@huawei.com>
Cc: peterz@infradead.org, mingo@redhat.com, acme@kernel.org,
	alexander.shishkin@linux.intel.com, mhiramat@kernel.org,
	jolsa@redhat.com, wangnan0@huawei.com, bintian.wang@huawei.com,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 1/3] perf tools: Use offset instead of dwarfnum in register table.
Date: Mon, 6 Feb 2017 13:02:29 +0000	[thread overview]
Message-ID: <20170206130229.GI19939@arm.com> (raw)
In-Reply-To: <589598D8.8050304@huawei.com>

On Sat, Feb 04, 2017 at 05:03:20PM +0800, Hekuang wrote:
> hi
> 
> 在 2017/2/3 21:00, Will Deacon 写道:
> >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@kernel.org>
> >>Signed-off-by: He Kuang <hekuang@huawei.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.
> 
> It's not undefined behaviour here,
> struct user_pt_regs {
>         __u64           regs[31];
>         __u64           sp;
>         __u64           pc;
>         __u64           pstate;
> };
> user_pt_regs->regs[31] is user_pt_regs->sp and the offset value is correct.

I think it's undefined from the C standard perspective.

> >
> >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
> I think add a ptregs_offset field is more suitable and makes the code
> indepent
> to struct user_pt_regs layout, for example if the structure changed to this:
> 
> struct user_pt_regs {
>         __u64           sp;
>         __u64           pc;
>         __u64           pstate;
>         __u64           regs[31];
> };

We won't reorder a uapi structure because that would binary compatibility.

Just send a patch using the dwarfnum as the index for the offset calculation,
like I've been saying since you posted the first version.

Will