perf symbols: Accept zero as the kernel base address

Commit-ID:  4b1303d0b01440f224cf81493b7e8e43d9b4965e
Gitweb:     http://git.kernel.org/tip/4b1303d0b01440f224cf81493b7e8e43d9b4965e
Author:     Arnaldo Carvalho de Melo <[email protected]>
AuthorDate: Tue, 11 Jul 2017 16:21:40 -0300
Committer:  Arnaldo Carvalho de Melo <[email protected]>
CommitDate: Wed, 12 Jul 2017 11:47:05 -0300

perf symbols: Accept zero as the kernel base address

Which is the case in S/390, where symbols were not being resolved because machine__get_kernel_start was only setting machine->kernel_start when the just successfully loaded kernel symtab had its map->start set to !0, when it was left at (1ULL << 63) assuming a partitioning of the address space for user/kernel, which is not the case in S/390 nor in Sparc.

So just check if map__load() was successfull and set machine->kernel_start to zero, fixing kernel symbol resolution on S/390.

Test performed by Thomas:

----

I like this patch. I have done a new build and removed all my debug output to start from scratch. Without your patch I get this:

# Samples: 4 of event 'cpu-clock'
# Event count (approx.): 1000000
#

  # Children      Self  Command  Shared Object     Symbol
  # ........  ........  .......  ................  ........................
      75.00%     0.00%  true     [unknown]         [k] 0x00000000004bedda
              |
              ---0x4bedda
                 |
                 |--50.00%--0x42693a
                 |          |
                 |           --25.00%--0x2a72e0
                 |                     0x2af0ca
                 |                     0x3d1003fe4c0
                 |
                  --25.00%--0x4272bc
                            0x26fa84

and with your patch (I just rebuilt the perf tool, nothing else and used the same perf.data file as input):

# Samples: 4 of event 'cpu-clock'
# Event count (approx.): 1000000
#

  # Children      Self  Command  Shared Object               Symbol
  # ........  ........  .......  ..........................  ..................................
      75.00%     0.00%  true     [kernel.vmlinux]            [k] pgm_check_handler
              |
              ---pgm_check_handler
                 do_dat_exception
                 handle_mm_fault
                 __handle_mm_fault
                 filemap_map_pages
                 |
                 |--25.00%--rcu_read_lock_held
                 |          rcu_lockdep_current_cpu_online
                 |          0x3d1003ff4c0
                 |
                  --25.00%--lock_release

Looks good to me....
----

---
 tools/perf/util/machine.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 5de2b86..2e9eb6a 100644
@@ -2209,7 +2209,7 @@ int machine__get_kernel_start(struct machine *machine)
 	machine->kernel_start = 1ULL << 63;
 	if (map) {
 		err = map__load(map);
-		if (map->start)
+		if (!err)
 			machine->kernel_start = map->start;
 	}
 	return err;