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

[Hekuang]

hi

å 2017/2/3 21:00, Will Deacon åé:
> 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@xxxxxxxxxx>
> > Signed-off-by: He Kuang <hekuang@xxxxxxxxxx>
> > ---
> >   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 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];
};

The multiply result will be incorrect.
Patch updated and the change is similar to commit "4679bccaa308"
 (perf tools powerpc: Add support for generating bpf prologue)

Please review, thanks.