linux-kernel - [tip:perf/urgent] perf/x86/intel: Add support/quirk for the MISPREDICT bit on Knights Landing CPUs

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Message-ID: <tip-16160c1946b702dcfa95ef63389a56deb2f1c7cb@git.kernel.org>
Date:   Mon, 10 Sep 2018 03:03:56 -0700
From:   tip-bot for Jacek Tomaka <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     jacek.tomaka@...zta.fm, mingo@...nel.org, peterz@...radead.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        torvalds@...ux-foundation.org, hpa@...or.com
Subject: [tip:perf/urgent] perf/x86/intel: Add support/quirk for the
 MISPREDICT bit on Knights Landing CPUs

Commit-ID:  16160c1946b702dcfa95ef63389a56deb2f1c7cb
Gitweb:     https://git.kernel.org/tip/16160c1946b702dcfa95ef63389a56deb2f1c7cb
Author:     Jacek Tomaka <jacek.tomaka@...zta.fm>
AuthorDate: Thu, 2 Aug 2018 09:38:30 +0800
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 10 Sep 2018 10:03:01 +0200

perf/x86/intel: Add support/quirk for the MISPREDICT bit on Knights Landing CPUs

Problem: perf did not show branch predicted/mispredicted bit in brstack.

Output of perf -F brstack for profile collected

Before:

 0x4fdbcd/0x4fdc03/-/-/-/0
 0x45f4c1/0x4fdba0/-/-/-/0
 0x45f544/0x45f4bb/-/-/-/0
 0x45f555/0x45f53c/-/-/-/0
 0x7f66901cc24b/0x45f555/-/-/-/0
 0x7f66901cc22e/0x7f66901cc23d/-/-/-/0
 0x7f66901cc1ff/0x7f66901cc20f/-/-/-/0
 0x7f66901cc1e8/0x7f66901cc1fc/-/-/-/0

After:

 0x4fdbcd/0x4fdc03/P/-/-/0
 0x45f4c1/0x4fdba0/P/-/-/0
 0x45f544/0x45f4bb/P/-/-/0
 0x45f555/0x45f53c/P/-/-/0
 0x7f66901cc24b/0x45f555/P/-/-/0
 0x7f66901cc22e/0x7f66901cc23d/P/-/-/0
 0x7f66901cc1ff/0x7f66901cc20f/P/-/-/0
 0x7f66901cc1e8/0x7f66901cc1fc/P/-/-/0

Cause:

As mentioned in Software Development Manual vol 3, 17.4.8.1,
IA32_PERF_CAPABILITIES[5:0] indicates the format of the address that is
stored in the LBR stack. Knights Landing reports 1 (LBR_FORMAT_LIP) as
its format. Despite that, registers containing FROM address of the branch,
do have MISPREDICT bit but because of the format indicated in
IA32_PERF_CAPABILITIES[5:0], LBR did not read MISPREDICT bit.

Solution:

Teach LBR about above Knights Landing quirk and make it read MISPREDICT bit.

Signed-off-by: Jacek Tomaka <jacek.tomaka@...zta.fm>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/20180802013830.10600-1-jacekt@dugeo.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/events/intel/lbr.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
index f3e006bed9a7..c88ed39582a1 100644
--- a/arch/x86/events/intel/lbr.c
+++ b/arch/x86/events/intel/lbr.c
@@ -1272,4 +1272,8 @@ void intel_pmu_lbr_init_knl(void)
 
 	x86_pmu.lbr_sel_mask = LBR_SEL_MASK;
 	x86_pmu.lbr_sel_map  = snb_lbr_sel_map;
+
+	/* Knights Landing does have MISPREDICT bit */
+	if (x86_pmu.intel_cap.lbr_format == LBR_FORMAT_LIP)
+		x86_pmu.intel_cap.lbr_format = LBR_FORMAT_EIP_FLAGS;
 }