[PATCH] perf core: Use KSTK_ESP() instead of pt_regs->sp while output user regs

[root <chenggang.qin@gmail.com>]

From: Chenggang Qin <chenggang.qcg@taobao.com>

For x86_64, the exact value of user stack's esp should be got by KSTK_ESP(current).
current->thread.usersp is copied from PDA while enter ring0. 
Now, we output the value of sp from pt_regs. But pt_regs->sp has changed before
it was pushed into kernel stack.

So, we cannot get the correct callchain while unwind some user stacks.
For example, if the stack contains __lll_unlock_wake()/__lll_lock_wait(), the
callchain will break some times with the latest version of libunwind.
The root cause is the sp that is used by libunwind may be wrong.

If we use KSTK_ESP(current), the correct callchain can be got everytime.
Other architectures also have KSTK_ESP() macro.

Signed-off-by: Chenggang Qin <chenggang.qcg@taobao.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Yanmin Zhang <yanmin.zhang@intel.com>

---
 arch/x86/kernel/perf_regs.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kernel/perf_regs.c b/arch/x86/kernel/perf_regs.c
index e309cc5..5da8df8 100644
--- a/arch/x86/kernel/perf_regs.c
+++ b/arch/x86/kernel/perf_regs.c
@@ -60,6 +60,9 @@ u64 perf_reg_value(struct pt_regs *regs, int idx)
 	if (WARN_ON_ONCE(idx >= ARRAY_SIZE(pt_regs_offset)))
 		return 0;
 
+	if (idx == PERF_REG_X86_SP)
+		return KSTK_ESP(current);
+
 	return regs_get_register(regs, pt_regs_offset[idx]);
 }
 
-- 
1.9.1