diff mbox series

[N/U,02/11] perf/x86/amd/lbr: Use freeze based on availability

Message ID 20240411063027.493165-3-andrea.righi@canonical.com
State New
Headers show
Series Apply mitigations for the native BHI hardware vulnerabilty | expand

Commit Message

Andrea Righi April 11, 2024, 6:24 a.m. UTC 
From: Sandipan Das <sandipan.das@amd.com>

BugLink: https://bugs.launchpad.net/bugs/2060909

Currently, the LBR code assumes that LBR Freeze is supported on all processors
when X86_FEATURE_AMD_LBR_V2 is available i.e. CPUID leaf 0x80000022[EAX]
bit 1 is set. This is incorrect as the availability of the feature is
additionally dependent on CPUID leaf 0x80000022[EAX] bit 2 being set,
which may not be set for all Zen 4 processors.

Define a new feature bit for LBR and PMC freeze and set the freeze enable bit
(FLBRI) in DebugCtl (MSR 0x1d9) conditionally.

It should still be possible to use LBR without freeze for profile-guided
optimization of user programs by using an user-only branch filter during
profiling. When the user-only filter is enabled, branches are no longer
recorded after the transition to CPL 0 upon PMI arrival. When branch
entries are read in the PMI handler, the branch stack does not change.

E.g.

  $ perf record -j any,u -e ex_ret_brn_tkn ./workload

Since the feature bit is visible under flags in /proc/cpuinfo, it can be
used to determine the feasibility of use-cases which require LBR Freeze
to be supported by the hardware such as profile-guided optimization of
kernels.

Fixes: ca5b7c0d9621 ("perf/x86/amd/lbr: Add LbrExtV2 branch record support")
Signed-off-by: Sandipan Das <sandipan.das@amd.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/69a453c97cfd11c6f2584b19f937fe6df741510f.1711091584.git.sandipan.das@amd.com
(cherry picked from commit 598c2fafc06fe5c56a1a415fb7b544b31453d637)
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
---
 arch/x86/events/amd/core.c         |  4 ++--
 arch/x86/events/amd/lbr.c          | 16 ++++++++++------
 arch/x86/include/asm/cpufeatures.h |  8 ++++++++
 arch/x86/kernel/cpu/scattered.c    |  1 +
 4 files changed, 21 insertions(+), 8 deletions(-)
diff mbox series

Patch

diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
index eb5c8539d657..81f6190e3b58 100644
--- a/arch/x86/events/amd/core.c
+++ b/arch/x86/events/amd/core.c
@@ -918,8 +918,8 @@  static int amd_pmu_v2_handle_irq(struct pt_regs *regs)
 	if (!status)
 		goto done;
 
-	/* Read branch records before unfreezing */
-	if (status & GLOBAL_STATUS_LBRS_FROZEN) {
+	/* Read branch records */
+	if (x86_pmu.lbr_nr) {
 		amd_pmu_lbr_read();
 		status &= ~GLOBAL_STATUS_LBRS_FROZEN;
 	}
diff --git a/arch/x86/events/amd/lbr.c b/arch/x86/events/amd/lbr.c
index eb31f850841a..110e34c59643 100644
--- a/arch/x86/events/amd/lbr.c
+++ b/arch/x86/events/amd/lbr.c
@@ -400,10 +400,12 @@  void amd_pmu_lbr_enable_all(void)
 		wrmsrl(MSR_AMD64_LBR_SELECT, lbr_select);
 	}
 
-	rdmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl);
-	rdmsrl(MSR_AMD_DBG_EXTN_CFG, dbg_extn_cfg);
+	if (cpu_feature_enabled(X86_FEATURE_AMD_LBR_PMC_FREEZE)) {
+		rdmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl);
+		wrmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl | DEBUGCTLMSR_FREEZE_LBRS_ON_PMI);
+	}
 
-	wrmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl | DEBUGCTLMSR_FREEZE_LBRS_ON_PMI);
+	rdmsrl(MSR_AMD_DBG_EXTN_CFG, dbg_extn_cfg);
 	wrmsrl(MSR_AMD_DBG_EXTN_CFG, dbg_extn_cfg | DBG_EXTN_CFG_LBRV2EN);
 }
 
@@ -416,10 +418,12 @@  void amd_pmu_lbr_disable_all(void)
 		return;
 
 	rdmsrl(MSR_AMD_DBG_EXTN_CFG, dbg_extn_cfg);
-	rdmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl);
-
 	wrmsrl(MSR_AMD_DBG_EXTN_CFG, dbg_extn_cfg & ~DBG_EXTN_CFG_LBRV2EN);
-	wrmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl & ~DEBUGCTLMSR_FREEZE_LBRS_ON_PMI);
+
+	if (cpu_feature_enabled(X86_FEATURE_AMD_LBR_PMC_FREEZE)) {
+		rdmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl);
+		wrmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl & ~DEBUGCTLMSR_FREEZE_LBRS_ON_PMI);
+	}
 }
 
 __init int amd_pmu_lbr_init(void)
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index ce2d9927da93..e3f71b9d43b9 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -456,6 +456,14 @@ 
 #define X86_FEATURE_IBPB_BRTYPE		(20*32+28) /* "" MSR_PRED_CMD[IBPB] flushes all branch type predictions */
 #define X86_FEATURE_SRSO_NO		(20*32+29) /* "" CPU is not affected by SRSO */
 
+/*
+ * Extended auxiliary flags: Linux defined - for features scattered in various
+ * CPUID levels like 0x80000022, etc.
+ *
+ * Reuse free bits when adding new feature flags!
+ */
+#define X86_FEATURE_AMD_LBR_PMC_FREEZE	(21*32+ 0) /* AMD LBR and PMC Freeze */
+
 /*
  * BUG word(s)
  */
diff --git a/arch/x86/kernel/cpu/scattered.c b/arch/x86/kernel/cpu/scattered.c
index 0dad49a09b7a..a515328d9d7d 100644
--- a/arch/x86/kernel/cpu/scattered.c
+++ b/arch/x86/kernel/cpu/scattered.c
@@ -49,6 +49,7 @@  static const struct cpuid_bit cpuid_bits[] = {
 	{ X86_FEATURE_BMEC,		CPUID_EBX,  3, 0x80000020, 0 },
 	{ X86_FEATURE_PERFMON_V2,	CPUID_EAX,  0, 0x80000022, 0 },
 	{ X86_FEATURE_AMD_LBR_V2,	CPUID_EAX,  1, 0x80000022, 0 },
+	{ X86_FEATURE_AMD_LBR_PMC_FREEZE,	CPUID_EAX,  2, 0x80000022, 0 },
 	{ 0, 0, 0, 0, 0 }
 };