[PATCH 13/13] perf/x86/amd/lbr: Add LbrExtV2 branch speculation info support

From: Sandipan Das
Date: Thu Aug 11 2022 - 08:39:53 EST


Provide branch speculation information captured via AMD Last Branch Record
Extension Version 2 (LbrExtV2) by setting the speculation info in branch
records. The info is based on the "valid" and "spec" bits in the Branch To
registers.

Suggested-by: Stephane Eranian <eranian@xxxxxxxxxx>
Signed-off-by: Sandipan Das <sandipan.das@xxxxxxx>
---
arch/x86/events/amd/lbr.c | 35 ++++++++++++++++++++++++++++++++---
1 file changed, 32 insertions(+), 3 deletions(-)

diff --git a/arch/x86/events/amd/lbr.c b/arch/x86/events/amd/lbr.c
index 5fa985cd44cb..3fd316b6adda 100644
--- a/arch/x86/events/amd/lbr.c
+++ b/arch/x86/events/amd/lbr.c
@@ -146,12 +146,19 @@ static void amd_pmu_lbr_filter(void)
}
}

+static const int lbr_spec_map[PERF_BR_SPEC_MAX] = {
+ PERF_BR_SPEC_NA,
+ PERF_BR_SPEC_WRONG_PATH,
+ PERF_BR_NON_SPEC_CORRECT_PATH,
+ PERF_BR_SPEC_CORRECT_PATH,
+};
+
void amd_pmu_lbr_read(void)
{
struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
struct perf_branch_entry *br = cpuc->lbr_entries;
struct branch_entry entry;
- int out = 0, i;
+ int out = 0, idx, i;

if (!cpuc->lbr_users)
return;
@@ -160,8 +167,11 @@ void amd_pmu_lbr_read(void)
entry.from.full = amd_pmu_lbr_get_from(i);
entry.to.full = amd_pmu_lbr_get_to(i);

- /* Check if a branch has been logged */
- if (!entry.to.split.valid)
+ /*
+ * Check if a branch has been logged; if valid = 0, spec = 0
+ * then no branch was recorded
+ */
+ if (!entry.to.split.valid && !entry.to.split.spec)
continue;

perf_clear_branch_entry_bitfields(br + out);
@@ -170,6 +180,25 @@ void amd_pmu_lbr_read(void)
br[out].to = sign_ext_branch_ip(entry.to.split.ip);
br[out].mispred = entry.from.split.mispredict;
br[out].predicted = !br[out].mispred;
+
+ /*
+ * Set branch speculation information using the status of
+ * the valid and spec bits.
+ *
+ * When valid = 0, spec = 0, no branch was recorded and the
+ * entry is discarded as seen above.
+ *
+ * When valid = 0, spec = 1, the recorded branch was
+ * speculative but took the wrong path.
+ *
+ * When valid = 1, spec = 0, the recorded branch was
+ * non-speculative but took the correct path.
+ *
+ * When valid = 1, spec = 1, the recorded branch was
+ * speculative and took the correct path
+ */
+ idx = (entry.to.split.valid << 1) | entry.to.split.spec;
+ br[out].spec = lbr_spec_map[idx];
out++;
}

--
2.34.1