perf/x86/intel: Introduce the fourth fixed counter

The fourth fixed counter, TOPDOWN.SLOTS, is introduced in Ice Lake to
measure the level 1 TopDown events.

Add MSR address and macros for the new fixed counter, which will be used
in a later patch.

Add comments to explain the event encoding rules for the fixed counters.
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200723171117.9918-4-kan.liang@linux.intel.com

arch/x86/include/asm/perf_event.h

@@ -197,12 +197,24 @@ 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: */
@@ -213,11 +225,16 @@ 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.
  *