linux-kernel - [PATCH 1/1] perf/x86/intel: fix LBR callstack issue caused by FREEZE_LBRS_ON

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Date:	Mon, 17 Aug 2015 08:37:31 -0400
From:	kan.liang@...el.com
To:	peterz@...radead.org
Cc:	mail@...ianw.de, ak@...ux.intel.com, eranian@...gle.com,
	linux-kernel@...r.kernel.org, Kan Liang <kan.liang@...el.com>
Subject: [PATCH 1/1] perf/x86/intel: fix LBR callstack issue caused by FREEZE_LBRS_ON_PMI

From: Kan Liang <kan.liang@...el.com>

This patch fixes an issue which introduced by commit
1a78d93750bb5f61abdc59a91fc3bd06a214542a ("perf/x86/intel: Streamline
LBR MSR handling in PMI").

The old patch not only avoids writing LBR_SELECT MSR in PMI, but also
avoids updating lbr_select variable. So in PMI, FREEZE_LBRS_ON_PMI bit
is always mistakenly set for IA32_DEBUGCTLMSR MSR, which causes
superfluous increase/decrease of LBR_TOS when collecting LBR callstack.

Reported-by: Milian Wolff <mail@...ianw.de>
Signed-off-by: Kan Liang <kan.liang@...el.com>
---
 arch/x86/kernel/cpu/perf_event_intel_lbr.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event_intel_lbr.c b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
index b2c9475..a1d07c7 100644
--- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
@@ -151,10 +151,9 @@ static void __intel_pmu_lbr_enable(bool pmi)
 	 * No need to reprogram LBR_SELECT in a PMI, as it
 	 * did not change.
 	 */
-	if (cpuc->lbr_sel && !pmi) {
-		lbr_select = cpuc->lbr_sel->config;
+	lbr_select = cpuc->lbr_sel->config;
+	if (cpuc->lbr_sel && !pmi)
 		wrmsrl(MSR_LBR_SELECT, lbr_select);
-	}
 
 	rdmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
 	orig_debugctl = debugctl;
-- 
1.8.3.1

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