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

From: Adrian Hunter <[email-masked]>
Date: Fri, 16 Aug 2019 16:00:02 +0300

On 16/08/19 4:45 AM, Leo Yan wrote:
> Hi Adrian,
> 
> On Thu, Aug 15, 2019 at 02:45:57PM +0300, Adrian Hunter wrote:
> 
> [...]
> 
>>>> 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();
>>
>> Doesn't it just need to loop through each kernel map to find the lowest
>> start address?
> 
> Based on your suggestion, I worked out below change and verified it
> can work well on arm64 for fixing up start address; please let me know
> if the change works for you?

How does that work if take a perf.data file to a machine with a different
architecture?

> 
> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> index f6ee7fbad3e4..51d78313dca1 100644
> --- a/tools/perf/util/machine.c
> +++ b/tools/perf/util/machine.c
> @@ -2671,9 +2671,26 @@ int machine__nr_cpus_avail(struct machine *machine)
>  	return machine ? perf_env__nr_cpus_avail(machine->env) : 0;
>  }
>  
> +static int machine__fixup_kernel_start(void *arg,
> +				       const char *name __maybe_unused,
> +				       char type,
> +				       u64 start)
> +{
> +	struct machine *machine = arg;
> +
> +	type = toupper(type);
> +
> +	/* Fixup for text, weak, data and bss sections. */
> +	if (type == 'T' || type == 'W' || type == 'D' || type == 'B')
> +		machine->kernel_start = min(machine->kernel_start, start);
> +
> +	return 0;
> +}
> +
>  int machine__get_kernel_start(struct machine *machine)
>  {
>  	struct map *map = machine__kernel_map(machine);
> +	char filename[PATH_MAX];
>  	int err = 0;
>  
>  	/*
> @@ -2687,6 +2704,7 @@ int machine__get_kernel_start(struct machine *machine)
>  	machine->kernel_start = 1ULL << 63;
>  	if (map) {
>  		err = map__load(map);
>  		/*
>  		 * On x86_64, PTI entry trampolines are less than the
>  		 * start of kernel text, but still above 2^63. So leave
> @@ -2695,6 +2713,16 @@ int machine__get_kernel_start(struct machine *machine)
>  		if (!err && !machine__is(machine, "x86_64"))
>  			machine->kernel_start = map->start;
>  	}
> +
> +	machine__get_kallsyms_filename(machine, filename, PATH_MAX);
> +
> +	if (symbol__restricted_filename(filename, "/proc/kallsyms"))
> +		goto out;
> +
> +	if (kallsyms__parse(filename, machine, machine__fixup_kernel_start))
> +		pr_warning("Fail to fixup kernel start address. skipping...\n");
> +
> +out:
>  	return err;
>  }
> 
> Thanks,
> Leo Yan
> 

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