perf/x86/intel: Fix MSR_LAST_BRANCH_FROM_x bug when no TSX

Intel's SDM states that bits 61:62 in MSR_LAST_BRANCH_FROM_x are the
TSX flags for formats with LBR_TSX flags (i.e. LBR_FORMAT_EIP_EFLAGS2).

However, when the CPU has TSX support deactivated, bits 61:62 actually
behave as follows:

  - For wrmsr(), bits 61:62 are considered part of the sign extension.
  - When capturing branches, the LBR hw will always clear bits 61:62.
    regardless of the sign extension.

Therefore, if:

  1) LBR has TSX format.
  2) CPU has no TSX support enabled.

... then any value passed to wrmsr() must be sign extended to 63 bits
and any value from rdmsr() must be converted to have a sign extension
of 61 bits, ignoring the values at TSX flags.

This bug was masked by the work-around to the Intel's CPU bug:
BJ94. "LBR May Contain Incorrect Information When Using FREEZE_LBRS_ON_PMI"
in Document Number: 324643-037US.

The aforementioned work-around uses hw flags to filter out all kernel
branches, limiting LBR callstack to user level execution only.

Since user addresses are not sign extended, they do not trigger the wrmsr()
bug in MSR_LAST_BRANCH_FROM_x when saved/restored at context switch.

To verify the hw bug:

  $ perf record -b -e cycles sleep 1
  $ rdmsr -p 0 0x680
  0x1fffffffb0b9b0cc
  $ wrmsr -p 0 0x680 0x1fffffffb0b9b0cc
  write(): Input/output error

The quirk for LBR_FROM_ MSRs is required before calls to wrmsrl() and
after rdmsrl().

This patch introduces it for wrmsrl()'s done for testing LBR support.

Future patch in series adds the quirk for context switch, that would
be required if LBR callstack is to be enabled for ring 0.

Signed-off-by: David Carrillo-Cisneros <davidcc@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Stephane Eranian <eranian@google.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Link: http://lkml.kernel.org/r/1466533874-52003-3-git-send-email-davidcc@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 52 insertions, 0 deletions

diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
index 2dca66cec617..88093e0915a9 100644
--- a/arch/x86/events/intel/lbr.c
+++ b/arch/x86/events/intel/lbr.c
@@ -81,6 +81,8 @@ static enum {
 #define LBR_FROM_FLAG_IN_TX    (1ULL << 62)
 #define LBR_FROM_FLAG_ABORT    (1ULL << 61)
 
+#define LBR_FROM_SIGNEXT_2MSB	(BIT_ULL(60) | BIT_ULL(59))
+
 /*
  * x86control flow change classification
  * x86control flow changes include branches, interrupts, traps, faults
@@ -235,6 +237,53 @@ enum {
 	LBR_VALID,
 };
 
+/*
+ * For formats with LBR_TSX flags (e.g. LBR_FORMAT_EIP_FLAGS2), bits 61:62 in
+ * MSR_LAST_BRANCH_FROM_x are the TSX flags when TSX is supported, but when
+ * TSX is not supported they have no consistent behavior:
+ *
+ *   - For wrmsr(), bits 61:62 are considered part of the sign extension.
+ *   - For HW updates (branch captures) bits 61:62 are always OFF and are not
+ *     part of the sign extension.
+ *
+ * Therefore, if:
+ *
+ *   1) LBR has TSX format
+ *   2) CPU has no TSX support enabled
+ *
+ * ... then any value passed to wrmsr() must be sign extended to 63 bits and any
+ * value from rdmsr() must be converted to have a 61 bits sign extension,
+ * ignoring the TSX flags.
+ */
+static inline bool lbr_from_signext_quirk_needed(void)
+{
+	int lbr_format = x86_pmu.intel_cap.lbr_format;
+	bool tsx_support = boot_cpu_has(X86_FEATURE_HLE) ||
+			   boot_cpu_has(X86_FEATURE_RTM);
+
+	return !tsx_support && (lbr_desc[lbr_format] & LBR_TSX);
+}
+
+DEFINE_STATIC_KEY_FALSE(lbr_from_quirk_key);
+
+/* If quirk is enabled, ensure sign extension is 63 bits: */
+inline u64 lbr_from_signext_quirk_wr(u64 val)
+{
+	if (static_branch_unlikely(&lbr_from_quirk_key)) {
+		/*
+		 * Sign extend into bits 61:62 while preserving bit 63.
+		 *
+		 * Quirk is enabled when TSX is disabled. Therefore TSX bits
+		 * in val are always OFF and must be changed to be sign
+		 * extension bits. Since bits 59:60 are guaranteed to be
+		 * part of the sign extension bits, we can just copy them
+		 * to 61:62.
+		 */
+		val |= (LBR_FROM_SIGNEXT_2MSB & val) << 2;
+	}
+	return val;
+}
+
 static void __intel_pmu_lbr_restore(struct x86_perf_task_context *task_ctx)
 {
 	int i;
@@ -1007,6 +1056,9 @@ void intel_pmu_lbr_init_hsw(void)
 
 	x86_pmu.lbr_sel_mask = LBR_SEL_MASK;
 	x86_pmu.lbr_sel_map  = hsw_lbr_sel_map;
+
+	if (lbr_from_signext_quirk_needed())
+		static_branch_enable(&lbr_from_quirk_key);
 }
 
 /* skylake */