diff --git a/arch/x86/include/asm/perf_event.h b/arch/x86/include/asm/perf_event.h
index 65f6cfb37a7c20a4f7f63f41f0422c51f831934b..ece7c182d26c4bc14c0419218dd478de0d171715 100644
--- a/arch/x86/include/asm/perf_event.h
+++ b/arch/x86/include/asm/perf_event.h
@@ -146,12 +146,24 @@ struct x86_pmu_capability {
  */
 
 /*
- * All 3 fixed-mode PMCs are configured via this single MSR:
+ * All the fixed-mode PMCs are configured via this single MSR:
  */
 #define MSR_ARCH_PERFMON_FIXED_CTR_CTRL	0x38d
 
 /*
- * The counts are available in three separate MSRs:
+ * There is no event-code assigned to the fixed-mode PMCs.
+ *
+ * For a fixed-mode PMC, which has an equivalent event on a general-purpose
+ * PMC, the event-code of the equivalent event is used for the fixed-mode PMC,
+ * e.g., Instr_Retired.Any and CPU_CLK_Unhalted.Core.
+ *
+ * For a fixed-mode PMC, which doesn't have an equivalent event, a
+ * pseudo-encoding is used, e.g., CPU_CLK_Unhalted.Ref and TOPDOWN.SLOTS.
+ * The pseudo event-code for a fixed-mode PMC must be 0x00.
+ * The pseudo umask-code is 0xX. The X equals the index of the fixed
+ * counter + 1, e.g., the fixed counter 2 has the pseudo-encoding 0x0300.
+ *
+ * The counts are available in separate MSRs:
  */
 
 /* Instr_Retired.Any: */
@@ -162,11 +174,16 @@ struct x86_pmu_capability {
 #define MSR_ARCH_PERFMON_FIXED_CTR1	0x30a
 #define INTEL_PMC_IDX_FIXED_CPU_CYCLES	(INTEL_PMC_IDX_FIXED + 1)
 
-/* CPU_CLK_Unhalted.Ref: */
+/* CPU_CLK_Unhalted.Ref: event=0x00,umask=0x3 (pseudo-encoding) */
 #define MSR_ARCH_PERFMON_FIXED_CTR2	0x30b
 #define INTEL_PMC_IDX_FIXED_REF_CYCLES	(INTEL_PMC_IDX_FIXED + 2)
 #define INTEL_PMC_MSK_FIXED_REF_CYCLES	(1ULL << INTEL_PMC_IDX_FIXED_REF_CYCLES)
 
+/* TOPDOWN.SLOTS: event=0x00,umask=0x4 (pseudo-encoding) */
+#define MSR_ARCH_PERFMON_FIXED_CTR3	0x30c
+#define INTEL_PMC_IDX_FIXED_SLOTS	(INTEL_PMC_IDX_FIXED + 3)
+#define INTEL_PMC_MSK_FIXED_SLOTS	(1ULL << INTEL_PMC_IDX_FIXED_SLOTS)
+
 /*
  * We model BTS tracing as another fixed-mode PMC.
  *