Re: [PATCH 1/3] perf tests: Introduce perf_regs_load function on ARM64

[Jean Pihet]

Hi Mark,

ping on this series, see comment below.

On 25 March 2014 16:23, Jean Pihet <jean.pihet@xxxxxxxxxx> wrote:
> Hi Mark,
>
> On 21 March 2014 16:11, Mark Rutland <mark.rutland@xxxxxxx> wrote:
>> Hi Jean,
>>
>> On Wed, Mar 19, 2014 at 09:42:33AM +0000, Jean Pihet wrote:
>>> Introducing perf_regs_load function, which is going
>>> to be used for dwarf unwind test in following patches.
>>>
>>> It takes single argument as a pointer to the regs dump
>>> buffer and populates it with current registers values, as
>>> expected by the perf built-in unwinding test.
>>>
>>> Signed-off-by: Jean Pihet <jean.pihet@xxxxxxxxxx>
>>> Cc: Steve Capper <steve.capper@xxxxxxxxxx>
>>> Cc: Corey Ashford <cjashfor@xxxxxxxxxxxxxxxxxx>
>>> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
>>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>>> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
>>> Cc: Paul Mackerras <paulus@xxxxxxxxx>
>>> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
>>> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxxxxxxxxxx>
>>> Cc: David Ahern <dsahern@xxxxxxxxx>
>>> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
>>> ---
>>>  tools/perf/arch/arm64/Makefile            |  1 +
>>>  tools/perf/arch/arm64/include/perf_regs.h |  2 ++
>>>  tools/perf/arch/arm64/tests/regs_load.S   | 39 +++++++++++++++++++++++++++++++
>>>  3 files changed, 42 insertions(+)
>>>  create mode 100644 tools/perf/arch/arm64/tests/regs_load.S
>>>
>>> diff --git a/tools/perf/arch/arm64/Makefile b/tools/perf/arch/arm64/Makefile
>>> index 67e9b3d..9b8f87e 100644
>>> --- a/tools/perf/arch/arm64/Makefile
>>> +++ b/tools/perf/arch/arm64/Makefile
>>> @@ -4,4 +4,5 @@ LIB_OBJS += $(OUTPUT)arch/$(ARCH)/util/dwarf-regs.o
>>>  endif
>>>  ifndef NO_LIBUNWIND
>>>  LIB_OBJS += $(OUTPUT)arch/$(ARCH)/util/unwind-libunwind.o
>>> +LIB_OBJS += $(OUTPUT)arch/$(ARCH)/tests/regs_load.o
>>>  endif
>>> diff --git a/tools/perf/arch/arm64/include/perf_regs.h b/tools/perf/arch/arm64/include/perf_regs.h
>>> index 2359546..1e052f1 100644
>>> --- a/tools/perf/arch/arm64/include/perf_regs.h
>>> +++ b/tools/perf/arch/arm64/include/perf_regs.h
>>> @@ -9,6 +9,8 @@
>>>  #define PERF_REG_IP  PERF_REG_ARM64_PC
>>>  #define PERF_REG_SP  PERF_REG_ARM64_SP
>>>
>>> +void perf_regs_load(u64 *regs);
>>> +
>>>  static inline const char *perf_reg_name(int id)
>>>  {
>>>       switch (id) {
>>> diff --git a/tools/perf/arch/arm64/tests/regs_load.S b/tools/perf/arch/arm64/tests/regs_load.S
>>> new file mode 100644
>>> index 0000000..92ab968
>>> --- /dev/null
>>> +++ b/tools/perf/arch/arm64/tests/regs_load.S
>>> @@ -0,0 +1,39 @@
>>> +#include <linux/linkage.h>
>>> +
>>> +/*
>>> + * Implementation of void perf_regs_load(u64 *regs);
>>> + *
>>> + * This functions fills in the 'regs' buffer from the actual registers values,
>>> + * in the way the perf built-in unwinding test expects them:
>>> + * - the PC at the time at the call to this function. Since this function
>>> + *   is called using a bl instruction, the PC value is taken from LR,
>>
>> Is it guaranteed that this function is always invoked with a branch with
>> link instruction, or is that just what current compiler versions are
>> doing? I couldn't see where we would get that guarantee from.
> The current compiler implements the call as a bl instruction.
>
>> If it is called with a branch with link, then the LR value will be the
>> PC at call time + 4, rather than just the exact PC at call time. If not
>> then we don't have a guaranteed relationship between the PC at call time
>> and the current LR value.
>>
>> If the only place that perf_regs_load is used is a single test which
>> doesn't care about the precise PC at the time of the call, then it's
>> probably OK to use the LR value, but we should be careful to document
>> what the faked-up PC actually is and how we expect it to be used.
> The code is only used by an unwinding test. The unwinding code
> resolves the function name from an address range found in the dwarf
> information so in principle it is ok to use the PC/LR at the time of
> the call to a function.
>
> Is the comment above OK or do you want an update of the code as well?
What do you think?

Regards,
Jean

>
>>
>>> + * - the current SP (not touched by this function),
>>> + * - the current value of LR is merely retrieved and stored because the
>>> + *   value before the call to this function is unknown at this time; it will
>>> + *   be unwound from the dwarf information in unwind__get_entries.
>>> + */
>>> +
>>> +.text
>>> +.type perf_regs_load,%function
>>> +ENTRY(perf_regs_load)
>>> +     stp x0,  x1,  [x0], #16 // store x0..x29
>>> +     stp x2,  x3,  [x0], #16
>>> +     stp x4,  x5,  [x0], #16
>>> +     stp x6,  x7,  [x0], #16
>>> +     stp x8,  x9,  [x0], #16
>>> +     stp x10, x11, [x0], #16
>>> +     stp x12, x13, [x0], #16
>>> +     stp x14, x15, [x0], #16
>>> +     stp x16, x17, [x0], #16
>>> +     stp x18, x19, [x0], #16
>>> +     stp x20, x21, [x0], #16
>>> +     stp x22, x23, [x0], #16
>>> +     stp x24, x25, [x0], #16
>>> +     stp x26, x27, [x0], #16
>>> +     stp x28, x29, [x0], #16
>>> +     mov x1,  sp
>>> +     stp x30, x1,  [x0], #16 // store lr and sp
>>> +     str x30, [x0]           // store pc as lr in order to skip the call
>>> +                             //  to this function
>>
>> It might be better to word this a "store the lr in place of the pc". To
>> me at least the current wording implies the opposite of what the code
>> seems to be doing.
> Ok the last comment can be updated.
>
> Thanks!
> Jean
>
>>
>> Cheers,
>> Mark.
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