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Subject[PATCH v2] perf/x86/intel: move regs->flags EXACT bit init
Date
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.

In V2, we improve reability of the comments.

Signed-off-by: Stephane Eranian <eranian@google.com>
---
arch/x86/events/intel/ds.c | 34 ++++++++++++++++++++++++----------
1 file changed, 24 insertions(+), 10 deletions(-)

diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
index da6780122786..e4e97fc8eec3 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 eventing IP
+ * which avoids the off-by-1 skid. 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)) &&
--
2.7.4
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