Subject: Re: [PATCH v5] perf machine: arm/arm64: Improve completeness for kernel address space

From: Leo Yan <[email-masked]>
Date: Thu, 15 Aug 2019 19:32:42 +0800

Hi Adrian,

On Thu, Aug 15, 2019 at 11:54:54AM +0300, Adrian Hunter wrote:

[...]

> > diff --git a/tools/perf/Makefile.config b/tools/perf/Makefile.config
> > index e4988f49ea79..d7ff839d8b20 100644
> > --- a/tools/perf/Makefile.config
> > +++ b/tools/perf/Makefile.config
> > @@ -48,9 +48,20 @@ ifeq ($(SRCARCH),x86)
> >    NO_PERF_REGS := 0
> >  endif
> >  
> > +ARM_PRE_START_SIZE := 0
> > +
> >  ifeq ($(SRCARCH),arm)
> >    NO_PERF_REGS := 0
> >    LIBUNWIND_LIBS = -lunwind -lunwind-arm
> > +  ifneq ($(wildcard $(srctree)/arch/$(SRCARCH)/kernel/vmlinux.lds),)
> > +    # Extract info from lds:
> > +    #   . = ((0xC0000000)) + 0x00208000;
> > +    # ARM_PRE_START_SIZE := 0x00208000
> > +    ARM_PRE_START_SIZE := $(shell egrep ' \. \= \({2}0x[0-9a-fA-F]+\){2}' \
> > +      $(srctree)/arch/$(SRCARCH)/kernel/vmlinux.lds | \
> > +      sed -e 's/[(|)|.|=|+|<|;|-]//g' -e 's/ \+/ /g' -e 's/^[ \t]*//' | \
> > +      awk -F' ' '{printf "0x%x", $$2}' 2>/dev/null)
> > +  endif
> >  endif
> >  
> >  ifeq ($(SRCARCH),arm64)
> > @@ -58,8 +69,19 @@ ifeq ($(SRCARCH),arm64)
> >    NO_SYSCALL_TABLE := 0
> >    CFLAGS += -I$(OUTPUT)arch/arm64/include/generated
> >    LIBUNWIND_LIBS = -lunwind -lunwind-aarch64
> > +  ifneq ($(wildcard $(srctree)/arch/$(SRCARCH)/kernel/vmlinux.lds),)
> > +    # Extract info from lds:
> > +    #  . = ((((((((0xffffffffffffffff)) - (((1)) << (48)) + 1) + (0)) + (0x08000000))) + (0x08000000))) + 0x00080000;
> > +    # ARM_PRE_START_SIZE := (0x08000000 + 0x08000000 + 0x00080000) = 0x10080000
> > +    ARM_PRE_START_SIZE := $(shell egrep ' \. \= \({8}0x[0-9a-fA-F]+\){2}' \
> > +      $(srctree)/arch/$(SRCARCH)/kernel/vmlinux.lds | \
> > +      sed -e 's/[(|)|.|=|+|<|;|-]//g' -e 's/ \+/ /g' -e 's/^[ \t]*//' | \
> > +      awk -F' ' '{printf "0x%x", $$6+$$7+$$8}' 2>/dev/null)
> > +  endif
> 
> So, that is not going to work if you take a perf.data file to a non-arm machine?

Yeah, this patch will only allow perf to work correctly when perf
run natively on arm/arm64, so it can resolve partial of the issue.

> How come you cannot use kallsyms to get the information?

Thanks for pointing out this.  Sorry I skipped your comment "I don't
know how you intend to calculate ARM_PRE_START_SIZE" when you reviewed
the patch v3, I should use that chance to elaborate the detailed idea
and so can get more feedback/guidance before procceed.

Actually, I have considered to use kallsyms when worked on the previous
patch set.

As mentioned in patch set v4's cover letter, I tried to implement
machine__create_extra_kernel_maps() for arm/arm64, the purpose is to
parse kallsyms so can find more kernel maps and thus also can fixup
the kernel start address.  But I found the 'perf script' tool directly
calls machine__get_kernel_start() instead of running into the flow for
machine__create_extra_kernel_maps(); so I finally gave up to use
machine__create_extra_kernel_maps() for tweaking kernel start address
and went back to use this patch's approach by parsing lds files.

So for next step, I want to get some guidances:

- One method is to add a new weak function, e.g.
  arch__fix_kernel_text_start(), then every arch can implement its own
  function to fixup the kernel start address;

  For arm/arm64, can use kallsyms to find the symbols with least
  address and fixup for kernel start address.

- Another method is to directly parse kallsyms in the function
  machine__get_kernel_start(), thus the change can be used for all
  archs;

Seems to me the second method is to address this issue as a common
issue crossing all archs.  But not sure if this is the requirement for
all archs or just this is only required for arm/arm64.  Please let me
know what's your preference or other thoughts.  Thanks a lot!

Leo.

> >  endif
> >  
> > +CFLAGS += -DARM_PRE_START_SIZE=$(ARM_PRE_START_SIZE)
> > +
> >  ifeq ($(SRCARCH),csky)
> >    NO_PERF_REGS := 0
> >  endif
> > diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> > index f6ee7fbad3e4..e993f891bb82 100644
> > --- a/tools/perf/util/machine.c
> > +++ b/tools/perf/util/machine.c
> > @@ -2687,13 +2687,26 @@ int machine__get_kernel_start(struct machine *machine)
> >  	machine->kernel_start = 1ULL << 63;
> >  	if (map) {
> >  		err = map__load(map);
> > +		if (err)
> > +			return err;
> > +
> >  		/*
> >  		 * On x86_64, PTI entry trampolines are less than the
> >  		 * start of kernel text, but still above 2^63. So leave
> >  		 * kernel_start = 1ULL << 63 for x86_64.
> >  		 */
> > -		if (!err && !machine__is(machine, "x86_64"))
> > +		if (!machine__is(machine, "x86_64"))
> >  			machine->kernel_start = map->start;
> > +
> > +		/*
> > +		 * On arm/arm64, the kernel uses some memory regions which are
> > +		 * prior to '_stext' symbol; to reflect the complete kernel
> > +		 * address space, compensate these pre-defined regions for
> > +		 * kernel start address.
> > +		 */
> > +		if (!strcmp(perf_env__arch(machine->env), "arm") ||
> > +		    !strcmp(perf_env__arch(machine->env), "arm64"))
> > +			machine->kernel_start -= ARM_PRE_START_SIZE;
> >  	}
> >  	return err;
> >  }
> > 
> 

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