linux-kernel - [PATCH v3 2/3] perf/x86/amd/lbr: Discard erroneous branch entries

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Date: Mon, 29 Jan 2024 16:36:25 +0530
From: Sandipan Das <sandipan.das@....com>
To: <linux-perf-users@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC: <x86@...nel.org>, <peterz@...radead.org>, <mingo@...hat.com>,
	<acme@...nel.org>, <mark.rutland@....com>,
	<alexander.shishkin@...ux.intel.com>, <jolsa@...nel.org>,
	<namhyung@...nel.org>, <adrian.hunter@...el.com>, <tglx@...utronix.de>,
	<bp@...en8.de>, <eranian@...gle.com>, <irogers@...gle.com>,
	<mario.limonciello@....com>, <ravi.bangoria@....com>,
	<ananth.narayan@....com>, <sandipan.das@....com>
Subject: [PATCH v3 2/3] perf/x86/amd/lbr: Discard erroneous branch entries

The Revision Guide for AMD Family 19h Model 10-1Fh processors, found at
the link below, declares Erratum 1452 which states that non-branch
entries may erroneously be recorded in the Last Branch Record (LBR)
stack with the valid and spec bits set. Such entries can be recognized
by inspecting bit 61 of the corresponding LastBranchStackToIp register.
This bit is currently reserved but if found to be set, the associated
branch entry should be discarded.

Link: https://bugzilla.kernel.org/attachment.cgi?id=305518
Signed-off-by: Sandipan Das <sandipan.das@....com>
---
 arch/x86/events/amd/lbr.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/events/amd/lbr.c b/arch/x86/events/amd/lbr.c
index 110e34c59643..43bf2dbcdb82 100644
--- a/arch/x86/events/amd/lbr.c
+++ b/arch/x86/events/amd/lbr.c
@@ -173,9 +173,11 @@ void amd_pmu_lbr_read(void)
 
 		/*
 		 * Check if a branch has been logged; if valid = 0, spec = 0
-		 * then no branch was recorded
+		 * then no branch was recorded; if reserved = 1 then an
+		 * erroneous branch was recorded (see erratum 1452)
 		 */
-		if (!entry.to.split.valid && !entry.to.split.spec)
+		if ((!entry.to.split.valid && !entry.to.split.spec) ||
+		    entry.to.split.reserved)
 			continue;
 
 		perf_clear_branch_entry_bitfields(br + out);
-- 
2.34.1