Re: [PATCH] [RFC] ARM: perf: allow tracing with kernel tracepoints events

[Will Deacon]

Hi Jean,

On Fri, May 16, 2014 at 04:01:16PM +0100, Jean Pihet wrote:
> When tracing with tracepoints events the IP and CPSR are set to 0,
> preventing the perf code to resolve the symbols:
> 
> ./perf record -e kmem:kmalloc cal
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.007 MB perf.data (~321 samples) ]
> 
> ./perf report
> Overhead Command Shared Object Symbol
> ........ ....... ............. ...........
> 40.78%   cal     [unknown]     [.]00000000
> 31.6%    cal     [unknown]     [.]00000000
> 
> The examination of the gathered samples (perf report -D) shows the IP
> is set to 0 and that the samples are considered as user space samples,
> while the IP should be set from the registers and the samples should be
> considered as kernel samples.
> 
> The fix is to implement perf_arch_fetch_caller_regs for ARM, which
> fills the necessary registers: ip, lr, sp and cpsr (used to check
> the user mode property of the samples).
> 
> Heavily inspired from arch/arm/include/asm/kexec.h.
> 
> Reported by Sneha Priya on linaro-dev, cf.
> http://lists.linaro.org/pipermail/linaro-dev/2014-May/017151.html
> 
> Signed-off-by: Jean Pihet <jean.pihet@xxxxxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Reported-by: Sneha Priya <sneha.cse@xxxxxxxxxxx>
> ---
>  arch/arm/include/asm/perf_event.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/arch/arm/include/asm/perf_event.h b/arch/arm/include/asm/perf_event.h
> index 7558775..d466e39 100644
> --- a/arch/arm/include/asm/perf_event.h
> +++ b/arch/arm/include/asm/perf_event.h
> @@ -26,6 +26,19 @@ struct pt_regs;
>  extern unsigned long perf_instruction_pointer(struct pt_regs *regs);
>  extern unsigned long perf_misc_flags(struct pt_regs *regs);
>  #define perf_misc_flags(regs)	perf_misc_flags(regs)
> +
> +#define perf_arch_fetch_caller_regs(regs, __ip) {	\
> +	instruction_pointer(regs)= (__ip);		\
> +	__asm__ __volatile__ (				\
> +		"mov	%[_ARM_sp], sp\n\t"		\
> +		"str	lr, %[_ARM_lr]\n\t"		\
> +		"mrs	%[_ARM_cpsr], cpsr\n\t"		\
> +		: [_ARM_cpsr] "=r" (regs->ARM_cpsr),	\
> +		  [_ARM_sp] "=r" (regs->ARM_sp),	\
> +		  [_ARM_lr] "=o" (regs->ARM_lr)		\
> +		: : "memory"				\
> +	);						\
> +}

Why do we need to save lr? If it's for unwinding, what about fp? Also, why
do you have a "memory" clobber and why is this block marked volatile?

Will
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