perf/x86/intel: Move regs->flags EXACT bit init

This patch removes a redundant store on regs->flags introduced
by commit:

  71eb9ee9596d ("perf/x86/intel: Fix linear IP of PEBS real_ip on Haswell and later CPUs")

We were clearing the PERF_EFLAGS_EXACT but it was overwritten by
regs->flags = pebs->flags later on.

The PERF_EFLAGS_EXACT is a software flag using bit 3 of regs->flags.
X86 marks this bit as Reserved. To make sure this bit is zero before
we do any IP processing, we clear it explicitly.

Patch also removes the following assignment:

	regs->flags = pebs->flags | (regs->flags & PERF_EFLAGS_VM);

Because there is no regs->flags to preserve anymore because
set_linear_ip() is not called until later.

Signed-off-by: Stephane Eranian <eranian@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: kan.liang@intel.com
Link: http://lkml.kernel.org/r/1522909791-32498-1-git-send-email-eranian@google.com
[ Improve capitalization, punctuation and clarity of comments. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 24 insertions, 10 deletions

diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
index da6780122786..8a10a045b57b 100644
--- a/arch/x86/events/intel/ds.c
+++ b/arch/x86/events/intel/ds.c
@@ -1153,7 +1153,6 @@ static void setup_pebs_sample_data(struct perf_event *event,
         if (pebs == NULL)
                 return;
 
-        regs->flags &= ~PERF_EFLAGS_EXACT;
         sample_type = event->attr.sample_type;
         dsrc = sample_type & PERF_SAMPLE_DATA_SRC;
 
@@ -1197,7 +1196,13 @@ static void setup_pebs_sample_data(struct perf_event *event,
          * and PMI.
          */
         *regs = *iregs;
-        regs->flags = pebs->flags;
+
+        /*
+         * Initialize regs_>flags from PEBS,
+         * Clear exact bit (which uses x86 EFLAGS Reserved bit 3),
+         * i.e., do not rely on it being zero:
+         */
+        regs->flags = pebs->flags & ~PERF_EFLAGS_EXACT;
 
         if (sample_type & PERF_SAMPLE_REGS_INTR) {
                 regs->ax = pebs->ax;
@@ -1217,10 +1222,6 @@ static void setup_pebs_sample_data(struct perf_event *event,
                         regs->sp = pebs->sp;
                 }
 
-                /*
-                 * Preserve PERF_EFLAGS_VM from set_linear_ip().
-                 */
-                regs->flags = pebs->flags | (regs->flags & PERF_EFLAGS_VM);
 #ifndef CONFIG_X86_32
                 regs->r8 = pebs->r8;
                 regs->r9 = pebs->r9;
@@ -1234,20 +1235,33 @@ static void setup_pebs_sample_data(struct perf_event *event,
         }
 
         if (event->attr.precise_ip > 1) {
-                /* Haswell and later have the eventing IP, so use it: */
+                /*
+                 * Haswell and later processors have an 'eventing IP'
+                 * (real IP) which fixes the off-by-1 skid in hardware.
+                 * Use it when precise_ip >= 2 :
+                 */
                 if (x86_pmu.intel_cap.pebs_format >= 2) {
                         set_linear_ip(regs, pebs->real_ip);
                         regs->flags |= PERF_EFLAGS_EXACT;
                 } else {
-                        /* Otherwise use PEBS off-by-1 IP: */
+                        /* Otherwise, use PEBS off-by-1 IP: */
                         set_linear_ip(regs, pebs->ip);
 
-                        /* ... and try to fix it up using the LBR entries: */
+                        /*
+                         * With precise_ip >= 2, try to fix up the off-by-1 IP
+                         * using the LBR. If successful, the fixup function
+                         * corrects regs->ip and calls set_linear_ip() on regs:
+                         */
                         if (intel_pmu_pebs_fixup_ip(regs))
                                 regs->flags |= PERF_EFLAGS_EXACT;
                 }
-        } else
+        } else {
+                /*
+                 * When precise_ip == 1, return the PEBS off-by-1 IP,
+                 * no fixup attempted:
+                 */
                 set_linear_ip(regs, pebs->ip);
+        }
 
 
         if ((sample_type & (PERF_SAMPLE_ADDR | PERF_SAMPLE_PHYS_ADDR)) &&