[PATCH 2/2] perf/x86/intel/pt: Allow disabling branch tracing

[Alexander Shishkin]

Now that Intel PT supports more types of trace content than just branch
tracing, it may be useful to allow the user to disable branch tracing
when it is not needed.

The special case is BDW, where not setting BranchEn is not supported.

This patch adds 'no_branch' event format string to PT events, which
will disable setting BranchEn bit in the hardware trace configuration.

Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
---
 arch/x86/events/intel/pt.c | 36 +++++++++++++++++++++++++++++++++++-
 arch/x86/events/intel/pt.h |  1 +
 2 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
index ece5fb06db..57478c2d0f 100644
--- a/arch/x86/events/intel/pt.c
+++ b/arch/x86/events/intel/pt.c
@@ -28,6 +28,7 @@
 #include <asm/insn.h>
 #include <asm/io.h>
 #include <asm/intel_pt.h>
+#include <asm/intel-family.h>
 
 #include "../perf_event.h"
 #include "pt.h"
@@ -105,6 +106,7 @@ PMU_FORMAT_ATTR(mtc,		"config:9"	);
 PMU_FORMAT_ATTR(tsc,		"config:10"	);
 PMU_FORMAT_ATTR(noretcomp,	"config:11"	);
 PMU_FORMAT_ATTR(ptw,		"config:12"	);
+PMU_FORMAT_ATTR(no_branch,	"config:13"	);
 PMU_FORMAT_ATTR(mtc_period,	"config:14-17"	);
 PMU_FORMAT_ATTR(cyc_thresh,	"config:19-22"	);
 PMU_FORMAT_ATTR(psb_period,	"config:24-27"	);
@@ -117,6 +119,7 @@ static struct attribute *pt_formats_attr[] = {
 	&format_attr_tsc.attr,
 	&format_attr_noretcomp.attr,
 	&format_attr_ptw.attr,
+	&format_attr_no_branch.attr,
 	&format_attr_mtc_period.attr,
 	&format_attr_cyc_thresh.attr,
 	&format_attr_psb_period.attr,
@@ -197,6 +200,19 @@ static int __init pt_pmu_hw_init(void)
 		pt_pmu.tsc_art_den = eax;
 	}
 
+	/* model-specific quirks */
+	switch (boot_cpu_data.x86_model) {
+	case INTEL_FAM6_BROADWELL_CORE:
+	case INTEL_FAM6_BROADWELL_XEON_D:
+	case INTEL_FAM6_BROADWELL_GT3E:
+	case INTEL_FAM6_BROADWELL_X:
+		/* not setting BRANCH_EN will #GP, erratum BDM106 */
+		pt_pmu.branch_en_always_on = true;
+		break;
+	default:
+		break;
+	}
+
 	if (boot_cpu_has(X86_FEATURE_VMX)) {
 		/*
 		 * Intel SDM, 36.5 "Tracing post-VMXON" says that
@@ -265,6 +281,7 @@ static int __init pt_pmu_hw_init(void)
 
 #define PT_CONFIG_MASK (RTIT_CTL_TSC_EN		| \
 			RTIT_CTL_DISRETC	| \
+			RTIT_CTL_BRANCH_EN	| \
 			RTIT_CTL_CYC_PSB	| \
 			RTIT_CTL_MTC		| \
 			RTIT_CTL_PWR_EVT_EN	| \
@@ -332,6 +349,10 @@ static bool pt_event_valid(struct perf_event *event)
 			return false;
 	}
 
+	/* trying to unset BRANCH_EN where it is not supported */
+	if (config & RTIT_CTL_BRANCH_EN && pt_pmu.branch_en_always_on)
+		return false;
+
 	return true;
 }
 
@@ -419,7 +440,20 @@ static void pt_config(struct perf_event *event)
 	}
 
 	reg = pt_config_filters(event);
-	reg |= RTIT_CTL_TOPA | RTIT_CTL_BRANCH_EN | RTIT_CTL_TRACEEN;
+	reg |= RTIT_CTL_TOPA | RTIT_CTL_TRACEEN;
+
+	/*
+	 * Previously, we had BRANCH_EN on by default, but now that PT has
+	 * grown features outside of branch tracing, it is useful to allow
+	 * the user to disable it. So, to keep compatibility, setting
+	 * BRANCH_EN bit in the event config (no_branch=1) will have the
+	 * reverse effect and *not* set BRANCH_EN in the hardware
+	 * configuration.
+	 */
+	if (!(event->attr.config & RTIT_CTL_BRANCH_EN))
+		reg |= RTIT_CTL_BRANCH_EN;
+	else
+		event->attr.config &= ~RTIT_CTL_BRANCH_EN;
 
 	if (!event->attr.exclude_kernel)
 		reg |= RTIT_CTL_OS;
diff --git a/arch/x86/events/intel/pt.h b/arch/x86/events/intel/pt.h
index 53473c21b5..93a3633e4c 100644
--- a/arch/x86/events/intel/pt.h
+++ b/arch/x86/events/intel/pt.h
@@ -110,6 +110,7 @@ struct pt_pmu {
 	struct pmu		pmu;
 	u32			caps[PT_CPUID_REGS_NUM * PT_CPUID_LEAVES];
 	bool			vmx;
+	bool			branch_en_always_on;
 	unsigned long		max_nonturbo_ratio;
 	unsigned int		tsc_art_num;
 	unsigned int		tsc_art_den;
-- 
2.11.0