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diff --git a/kern/perfmon_types.h b/kern/perfmon_types.h
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+/*
+ * Copyright (c) 2014-2018 Remy Noel.
+ * Copyright (c) 2014-2018 Richard Braun.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ *
+ * Isolated type definitions used to avoid inclusion circular dependencies.
+ */
+
+#ifndef KERN_PERFMON_TYPES_H
+#define KERN_PERFMON_TYPES_H
+
+#ifdef CONFIG_PERFMON
+
+#include <stdbool.h>
+#include <stdint.h>
+
+#include <kern/spinlock_types.h>
+
+/*
+ * Maximum number of supported hardware counters.
+ */
+#define PERFMON_MAX_PMCS CONFIG_PERFMON_MAX_PMCS
+
+/*
+ * Performance monitoring event.
+ *
+ * An event may be unattached, attached to a thread, or attached to a CPU.
+ * When it is loaded, the current value of the underlying PMC is saved.
+ * When it is updated, the delta between the current and saved PMC values
+ * is added to the event value.
+ */
+struct perfmon_event {
+    struct spinlock lock;
+    unsigned int flags;
+    unsigned int id;
+    uint64_t pmc_value;
+    uint64_t value;
+
+    union {
+        struct thread *thread;
+        unsigned int cpu;
+    };
+
+    unsigned int pmc_index;
+};
+
+/*
+ * Per-thread performance monitoring counter.
+ *
+ * Per-thread PMCs are indexed the same way as global PMCs.
+ *
+ * A per-thread PMC is referenced when an event is attached to a thread.
+ * The PMC may only be loaded if the thread is running on a processor,
+ * as a result of an event being attached to the thread, or the thread
+ * being dispatched by the scheduler. Note that this allows a transient
+ * state to be seen where a per-thread PMC is both unused and loaded.
+ * This happens after detaching an event from a thread, resulting in
+ * the underlying per-thread PMC to become unused, but if the thread
+ * is running concurrently, the counter is still loaded. The implementation
+ * resolves the situation by unloading the counter, which is either
+ * done by an explicit unload cross-call, or when the scheduler preempts
+ * the thread and unloads its thread data.
+ *
+ * When a per-thread PMC is loaded, the current value of the underlying
+ * PMC is saved, and when it's updated, the delta between the current
+ * and saved PMC values is added to the per-thread PMC value.
+ */
+struct perfmon_td_pmc {
+    unsigned int nr_refs;
+    bool loaded;
+    unsigned int pmc_id;
+    unsigned int raw_event_id;
+    uint64_t cpu_pmc_value;
+    uint64_t value;
+};
+
+/*
+ * Per-thread performance monitoring data.
+ *
+ * Interrupts must be disabled when locking thread data.
+ */
+struct perfmon_td {
+    struct spinlock lock;
+    struct perfmon_td_pmc pmcs[PERFMON_MAX_PMCS];
+};
+
+#endif /* CONFIG_PERFMON */
+
+#endif /* KERN_PERFMON_TYPES_H */