Re: [PATCH v4 1/2] perf machine: Support arch's specific kernel start address

[Adrian Hunter]

On 10/08/19 10:21 AM, Leo Yan wrote:
> machine__get_kernel_start() gives out the kernel start address; some
> architectures need to tweak the start address so that can reflect the
> kernel start address correctly.  This is not only for x86_64 arch, but
> it is also required by other architectures, e.g. arm/arm64 needs to
> tweak the kernel start address so can include the kernel memory regions
> which are used before the '_stext' symbol.
> 
> This patch refactors machine__get_kernel_start() by adding a weak
> arch__fix_kernel_text_start(), any architecture can implement it to
> tweak its specific start address; this also allows the arch specific
> code to be placed into 'arch' folder.
> 
> Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
> ---
>  tools/perf/arch/x86/util/machine.c | 10 ++++++++++
>  tools/perf/util/machine.c          | 13 +++++++------
>  tools/perf/util/machine.h          |  2 ++
>  3 files changed, 19 insertions(+), 6 deletions(-)
> 
> diff --git a/tools/perf/arch/x86/util/machine.c b/tools/perf/arch/x86/util/machine.c
> index 1e9ec783b9a1..9f012131534a 100644
> --- a/tools/perf/arch/x86/util/machine.c
> +++ b/tools/perf/arch/x86/util/machine.c
> @@ -101,4 +101,14 @@ int machine__create_extra_kernel_maps(struct machine *machine,
>  	return ret;
>  }
>  
> +void arch__fix_kernel_text_start(u64 *start)
> +{
> +	/*
> +	 * 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.
> +	 */
> +	*start = 1ULL << 63;
> +}

That is needed for reporting x86 data on any arch i.e. it is not specific to
the compile-time architecture, it is specific to the perf.data file
architecture, which is what machine__is() compares. So, this looks wrong.

> +
>  #endif
> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> index f6ee7fbad3e4..603518835692 100644
> --- a/tools/perf/util/machine.c
> +++ b/tools/perf/util/machine.c
> @@ -2671,6 +2671,10 @@ int machine__nr_cpus_avail(struct machine *machine)
>  	return machine ? perf_env__nr_cpus_avail(machine->env) : 0;
>  }
>  
> +void __weak arch__fix_kernel_text_start(u64 *start __maybe_unused)
> +{
> +}
> +
>  int machine__get_kernel_start(struct machine *machine)
>  {
>  	struct map *map = machine__kernel_map(machine);
> @@ -2687,14 +2691,11 @@ 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
> -		 * kernel_start = 1ULL << 63 for x86_64.
> -		 */
> -		if (!err && !machine__is(machine, "x86_64"))
> +		if (!err)
>  			machine->kernel_start = map->start;
>  	}
> +
> +	arch__fix_kernel_text_start(&machine->kernel_start);
>  	return err;
>  }
>  
> diff --git a/tools/perf/util/machine.h b/tools/perf/util/machine.h
> index ef803f08ae12..9cb459f4bfbc 100644
> --- a/tools/perf/util/machine.h
> +++ b/tools/perf/util/machine.h
> @@ -278,6 +278,8 @@ void machine__get_kallsyms_filename(struct machine *machine, char *buf,
>  int machine__create_extra_kernel_maps(struct machine *machine,
>  				      struct dso *kernel);
>  
> +void arch__fix_kernel_text_start(u64 *start);
> +
>  /* Kernel-space maps for symbols that are outside the main kernel map and module maps */
>  struct extra_kernel_map {
>  	u64 start;
>