RISC-V: Preliminary Perf Support

The RISC-V ISA defines a core set of performance counters that must
exist on all processors along with a standard way to add more
performance counters.

This patch set adds preliminary perf support for RISC-V systems.  Long
term we'll move to model where all PMUs can be built into the kernel at
the same time, detected at runtime (possibly via device tree), and
provided to userspace.  Since we currently only support the ISA-mandated
performance counters there's no need to detect anything right now.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

6 files changed, 835 insertions, 0 deletions

diff --git a/Documentation/riscv/pmu.txt b/Documentation/riscv/pmu.txt
new file mode 100644
index 000000000000..b29f03a6d82f
--- /dev/null
+++ b/Documentation/riscv/pmu.txt
@@ -0,0 +1,249 @@
+Supporting PMUs on RISC-V platforms
+==========================================
+Alan Kao <alankao@andestech.com>, Mar 2018
+
+Introduction
+------------
+
+As of this writing, perf_event-related features mentioned in The RISC-V ISA
+Privileged Version 1.10 are as follows:
+(please check the manual for more details)
+
+* [m|s]counteren
+* mcycle[h], cycle[h]
+* minstret[h], instret[h]
+* mhpeventx, mhpcounterx[h]
+
+With such function set only, porting perf would require a lot of work, due to
+the lack of the following general architectural performance monitoring features:
+
+* Enabling/Disabling counters
+  Counters are just free-running all the time in our case.
+* Interrupt caused by counter overflow
+  No such feature in the spec.
+* Interrupt indicator
+  It is not possible to have many interrupt ports for all counters, so an
+  interrupt indicator is required for software to tell which counter has
+  just overflowed.
+* Writing to counters
+  There will be an SBI to support this since the kernel cannot modify the
+  counters [1].  Alternatively, some vendor considers to implement
+  hardware-extension for M-S-U model machines to write counters directly.
+
+This document aims to provide developers a quick guide on supporting their
+PMUs in the kernel.  The following sections briefly explain perf' mechanism
+and todos.
+
+You may check previous discussions here [1][2].  Also, it might be helpful
+to check the appendix for related kernel structures.
+
+
+1. Initialization
+-----------------
+
+*riscv_pmu* is a global pointer of type *struct riscv_pmu*, which contains
+various methods according to perf's internal convention and PMU-specific
+parameters.  One should declare such instance to represent the PMU.  By default,
+*riscv_pmu* points to a constant structure *riscv_base_pmu*, which has very
+basic support to a baseline QEMU model.
+
+Then he/she can either assign the instance's pointer to *riscv_pmu* so that
+the minimal and already-implemented logic can be leveraged, or invent his/her
+own *riscv_init_platform_pmu* implementation.
+
+In other words, existing sources of *riscv_base_pmu* merely provide a
+reference implementation.  Developers can flexibly decide how many parts they
+can leverage, and in the most extreme case, they can customize every function
+according to their needs.
+
+
+2. Event Initialization
+-----------------------
+
+When a user launches a perf command to monitor some events, it is first
+interpreted by the userspace perf tool into multiple *perf_event_open*
+system calls, and then each of them calls to the body of *event_init*
+member function that was assigned in the previous step.  In *riscv_base_pmu*'s
+case, it is *riscv_event_init*.
+
+The main purpose of this function is to translate the event provided by user
+into bitmap, so that HW-related control registers or counters can directly be
+manipulated.  The translation is based on the mappings and methods provided in
+*riscv_pmu*.
+
+Note that some features can be done in this stage as well:
+
+(1) interrupt setting, which is stated in the next section;
+(2) privilege level setting (user space only, kernel space only, both);
+(3) destructor setting.  Normally it is sufficient to apply *riscv_destroy_event*;
+(4) tweaks for non-sampling events, which will be utilized by functions such as
+*perf_adjust_period*, usually something like the follows:
+
+if (!is_sampling_event(event)) {
+        hwc->sample_period = x86_pmu.max_period;
+        hwc->last_period = hwc->sample_period;
+        local64_set(&hwc->period_left, hwc->sample_period);
+}
+
+In the case of *riscv_base_pmu*, only (3) is provided for now.
+
+
+3. Interrupt
+------------
+
+3.1. Interrupt Initialization
+
+This often occurs at the beginning of the *event_init* method. In common
+practice, this should be a code segment like
+
+int x86_reserve_hardware(void)
+{
+        int err = 0;
+
+        if (!atomic_inc_not_zero(&pmc_refcount)) {
+                mutex_lock(&pmc_reserve_mutex);
+                if (atomic_read(&pmc_refcount) == 0) {
+                        if (!reserve_pmc_hardware())
+                                err = -EBUSY;
+                        else
+                                reserve_ds_buffers();
+                }
+                if (!err)
+                        atomic_inc(&pmc_refcount);
+                mutex_unlock(&pmc_reserve_mutex);
+        }
+
+        return err;
+}
+
+And the magic is in *reserve_pmc_hardware*, which usually does atomic
+operations to make implemented IRQ accessible from some global function pointer.
+*release_pmc_hardware* serves the opposite purpose, and it is used in event
+destructors mentioned in previous section.
+
+(Note: From the implementations in all the architectures, the *reserve/release*
+pair are always IRQ settings, so the *pmc_hardware* seems somehow misleading.
+It does NOT deal with the binding between an event and a physical counter,
+which will be introduced in the next section.)
+
+3.2. IRQ Structure
+
+Basically, a IRQ runs the following pseudo code:
+
+for each hardware counter that triggered this overflow
+
+    get the event of this counter
+
+    // following two steps are defined as *read()*,
+    // check the section Reading/Writing Counters for details.
+    count the delta value since previous interrupt
+    update the event->count (# event occurs) by adding delta, and
+               event->hw.period_left by subtracting delta
+
+    if the event overflows
+        sample data
+        set the counter appropriately for the next overflow
+
+        if the event overflows again
+            too frequently, throttle this event
+        fi
+    fi
+
+end for
+
+However as of this writing, none of the RISC-V implementations have designed an
+interrupt for perf, so the details are to be completed in the future.
+
+4. Reading/Writing Counters
+---------------------------
+
+They seem symmetric but perf treats them quite differently.  For reading, there
+is a *read* interface in *struct pmu*, but it serves more than just reading.
+According to the context, the *read* function not only reads the content of the
+counter (event->count), but also updates the left period to the next interrupt
+(event->hw.period_left).
+
+But the core of perf does not need direct write to counters.  Writing counters
+is hidden behind the abstraction of 1) *pmu->start*, literally start counting so one
+has to set the counter to a good value for the next interrupt; 2) inside the IRQ
+it should set the counter to the same resonable value.
+
+Reading is not a problem in RISC-V but writing would need some effort, since
+counters are not allowed to be written by S-mode.
+
+
+5. add()/del()/start()/stop()
+-----------------------------
+
+Basic idea: add()/del() adds/deletes events to/from a PMU, and start()/stop()
+starts/stop the counter of some event in the PMU.  All of them take the same
+arguments: *struct perf_event *event* and *int flag*.
+
+Consider perf as a state machine, then you will find that these functions serve
+as the state transition process between those states.
+Three states (event->hw.state) are defined:
+
+* PERF_HES_STOPPED:	the counter is stopped
+* PERF_HES_UPTODATE:	the event->count is up-to-date
+* PERF_HES_ARCH:	arch-dependent usage ... we don't need this for now
+
+A normal flow of these state transitions are as follows:
+
+* A user launches a perf event, resulting in calling to *event_init*.
+* When being context-switched in, *add* is called by the perf core, with a flag
+  PERF_EF_START, which means that the event should be started after it is added.
+  At this stage, a general event is bound to a physical counter, if any.
+  The state changes to PERF_HES_STOPPED and PERF_HES_UPTODATE, because it is now
+  stopped, and the (software) event count does not need updating.
+** *start* is then called, and the counter is enabled.
+   With flag PERF_EF_RELOAD, it writes an appropriate value to the counter (check
+   previous section for detail).
+   Nothing is written if the flag does not contain PERF_EF_RELOAD.
+   The state now is reset to none, because it is neither stopped nor updated
+   (the counting already started)
+* When being context-switched out, *del* is called.  It then checks out all the
+  events in the PMU and calls *stop* to update their counts.
+** *stop* is called by *del*
+   and the perf core with flag PERF_EF_UPDATE, and it often shares the same
+   subroutine as *read* with the same logic.
+   The state changes to PERF_HES_STOPPED and PERF_HES_UPTODATE, again.
+
+** Life cycle of these two pairs: *add* and *del* are called repeatedly as
+  tasks switch in-and-out; *start* and *stop* is also called when the perf core
+  needs a quick stop-and-start, for instance, when the interrupt period is being
+  adjusted.
+
+Current implementation is sufficient for now and can be easily extended to
+features in the future.
+
+A. Related Structures
+---------------------
+
+* struct pmu: include/linux/perf_event.h
+* struct riscv_pmu: arch/riscv/include/asm/perf_event.h
+
+  Both structures are designed to be read-only.
+
+  *struct pmu* defines some function pointer interfaces, and most of them take
+*struct perf_event* as a main argument, dealing with perf events according to
+perf's internal state machine (check kernel/events/core.c for details).
+
+  *struct riscv_pmu* defines PMU-specific parameters.  The naming follows the
+convention of all other architectures.
+
+* struct perf_event: include/linux/perf_event.h
+* struct hw_perf_event
+
+  The generic structure that represents perf events, and the hardware-related
+details.
+
+* struct riscv_hw_events: arch/riscv/include/asm/perf_event.h
+
+  The structure that holds the status of events, has two fixed members:
+the number of events and the array of the events.
+
+References
+----------
+
+[1] https://github.com/riscv/riscv-linux/pull/124
+[2] https://groups.google.com/a/groups.riscv.org/forum/#!topic/sw-dev/f19TmCNP6yA
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index cd4fd85fde84..4495604394e5 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -25,6 +25,7 @@ config RISCV
 	select HAVE_DMA_API_DEBUG
 	select HAVE_DMA_CONTIGUOUS
 	select HAVE_GENERIC_DMA_COHERENT
+	select HAVE_PERF_EVENTS
 	select IRQ_DOMAIN
 	select NO_BOOTMEM
 	select RISCV_ISA_A if SMP
@@ -198,6 +199,19 @@ config RISCV_ISA_C
 config RISCV_ISA_A
 	def_bool y
 
+menu "supported PMU type"
+	depends on PERF_EVENTS
+
+config RISCV_BASE_PMU
+	bool "Base Performance Monitoring Unit"
+	def_bool y
+	help
+	  A base PMU that serves as a reference implementation and has limited
+	  feature of perf.  It can run on any RISC-V machines so serves as the
+	  fallback, but this option can also be disable to reduce kernel size.
+
+endmenu
+
 endmenu
 
 menu "Kernel type"
diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/Kbuild
index 4286a5f83876..576ffdca06ba 100644
--- a/arch/riscv/include/asm/Kbuild
+++ b/arch/riscv/include/asm/Kbuild
@@ -25,6 +25,7 @@ generic-y += kdebug.h
 generic-y += kmap_types.h
 generic-y += kvm_para.h
 generic-y += local.h
+generic-y += local64.h
 generic-y += mm-arch-hooks.h
 generic-y += mman.h
 generic-y += module.h
diff --git a/arch/riscv/include/asm/perf_event.h b/arch/riscv/include/asm/perf_event.h
new file mode 100644
index 000000000000..0e638a0c3feb
--- /dev/null
+++ b/arch/riscv/include/asm/perf_event.h
@@ -0,0 +1,84 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2018 SiFive
+ * Copyright (C) 2018 Andes Technology Corporation
+ *
+ */
+
+#ifndef _ASM_RISCV_PERF_EVENT_H
+#define _ASM_RISCV_PERF_EVENT_H
+
+#include <linux/perf_event.h>
+#include <linux/ptrace.h>
+
+#define RISCV_BASE_COUNTERS	2
+
+/*
+ * The RISCV_MAX_COUNTERS parameter should be specified.
+ */
+
+#ifdef CONFIG_RISCV_BASE_PMU
+#define RISCV_MAX_COUNTERS	2
+#endif
+
+#ifndef RISCV_MAX_COUNTERS
+#error "Please provide a valid RISCV_MAX_COUNTERS for the PMU."
+#endif
+
+/*
+ * These are the indexes of bits in counteren register *minus* 1,
+ * except for cycle.  It would be coherent if it can directly mapped
+ * to counteren bit definition, but there is a *time* register at
+ * counteren[1].  Per-cpu structure is scarce resource here.
+ *
+ * According to the spec, an implementation can support counter up to
+ * mhpmcounter31, but many high-end processors has at most 6 general
+ * PMCs, we give the definition to MHPMCOUNTER8 here.
+ */
+#define RISCV_PMU_CYCLE		0
+#define RISCV_PMU_INSTRET	1
+#define RISCV_PMU_MHPMCOUNTER3	2
+#define RISCV_PMU_MHPMCOUNTER4	3
+#define RISCV_PMU_MHPMCOUNTER5	4
+#define RISCV_PMU_MHPMCOUNTER6	5
+#define RISCV_PMU_MHPMCOUNTER7	6
+#define RISCV_PMU_MHPMCOUNTER8	7
+
+#define RISCV_OP_UNSUPP		(-EOPNOTSUPP)
+
+struct cpu_hw_events {
+	/* # currently enabled events*/
+	int			n_events;
+	/* currently enabled events */
+	struct perf_event	*events[RISCV_MAX_COUNTERS];
+	/* vendor-defined PMU data */
+	void			*platform;
+};
+
+struct riscv_pmu {
+	struct pmu	*pmu;
+
+	/* generic hw/cache events table */
+	const int	*hw_events;
+	const int	(*cache_events)[PERF_COUNT_HW_CACHE_MAX]
+				       [PERF_COUNT_HW_CACHE_OP_MAX]
+				       [PERF_COUNT_HW_CACHE_RESULT_MAX];
+	/* method used to map hw/cache events */
+	int		(*map_hw_event)(u64 config);
+	int		(*map_cache_event)(u64 config);
+
+	/* max generic hw events in map */
+	int		max_events;
+	/* number total counters, 2(base) + x(general) */
+	int		num_counters;
+	/* the width of the counter */
+	int		counter_width;
+
+	/* vendor-defined PMU features */
+	void		*platform;
+
+	irqreturn_t	(*handle_irq)(int irq_num, void *dev);
+	int		irq;
+};
+
+#endif /* _ASM_RISCV_PERF_EVENT_H */
diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
index 8586dd96c2f0..e1274fc03af4 100644
--- a/arch/riscv/kernel/Makefile
+++ b/arch/riscv/kernel/Makefile
@@ -39,4 +39,6 @@ obj-$(CONFIG_MODULE_SECTIONS)	+= module-sections.o
 obj-$(CONFIG_FUNCTION_TRACER)	+= mcount.o ftrace.o
 obj-$(CONFIG_DYNAMIC_FTRACE)	+= mcount-dyn.o
 
+obj-$(CONFIG_PERF_EVENTS)      += perf_event.o
+
 clean:
diff --git a/arch/riscv/kernel/perf_event.c b/arch/riscv/kernel/perf_event.c
new file mode 100644
index 000000000000..b0e10c4e9f77
--- /dev/null
+++ b/arch/riscv/kernel/perf_event.c
@@ -0,0 +1,485 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2008 Thomas Gleixner <tglx@linutronix.de>
+ * Copyright (C) 2008-2009 Red Hat, Inc., Ingo Molnar
+ * Copyright (C) 2009 Jaswinder Singh Rajput
+ * Copyright (C) 2009 Advanced Micro Devices, Inc., Robert Richter
+ * Copyright (C) 2008-2009 Red Hat, Inc., Peter Zijlstra
+ * Copyright (C) 2009 Intel Corporation, <markus.t.metzger@intel.com>
+ * Copyright (C) 2009 Google, Inc., Stephane Eranian
+ * Copyright 2014 Tilera Corporation. All Rights Reserved.
+ * Copyright (C) 2018 Andes Technology Corporation
+ *
+ * Perf_events support for RISC-V platforms.
+ *
+ * Since the spec. (as of now, Priv-Spec 1.10) does not provide enough
+ * functionality for perf event to fully work, this file provides
+ * the very basic framework only.
+ *
+ * For platform portings, please check Documentations/riscv/pmu.txt.
+ *
+ * The Copyright line includes x86 and tile ones.
+ */
+
+#include <linux/kprobes.h>
+#include <linux/kernel.h>
+#include <linux/kdebug.h>
+#include <linux/mutex.h>
+#include <linux/bitmap.h>
+#include <linux/irq.h>
+#include <linux/interrupt.h>
+#include <linux/perf_event.h>
+#include <linux/atomic.h>
+#include <linux/of.h>
+#include <asm/perf_event.h>
+
+static const struct riscv_pmu *riscv_pmu __read_mostly;
+static DEFINE_PER_CPU(struct cpu_hw_events, cpu_hw_events);
+
+/*
+ * Hardware & cache maps and their methods
+ */
+
+static const int riscv_hw_event_map[] = {
+	[PERF_COUNT_HW_CPU_CYCLES]		= RISCV_PMU_CYCLE,
+	[PERF_COUNT_HW_INSTRUCTIONS]		= RISCV_PMU_INSTRET,
+	[PERF_COUNT_HW_CACHE_REFERENCES]	= RISCV_OP_UNSUPP,
+	[PERF_COUNT_HW_CACHE_MISSES]		= RISCV_OP_UNSUPP,
+	[PERF_COUNT_HW_BRANCH_INSTRUCTIONS]	= RISCV_OP_UNSUPP,
+	[PERF_COUNT_HW_BRANCH_MISSES]		= RISCV_OP_UNSUPP,
+	[PERF_COUNT_HW_BUS_CYCLES]		= RISCV_OP_UNSUPP,
+};
+
+#define C(x) PERF_COUNT_HW_CACHE_##x
+static const int riscv_cache_event_map[PERF_COUNT_HW_CACHE_MAX]
+[PERF_COUNT_HW_CACHE_OP_MAX]
+[PERF_COUNT_HW_CACHE_RESULT_MAX] = {
+	[C(L1D)] = {
+		[C(OP_READ)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+		[C(OP_WRITE)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+		[C(OP_PREFETCH)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+	},
+	[C(L1I)] = {
+		[C(OP_READ)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+		[C(OP_WRITE)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+		[C(OP_PREFETCH)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+	},
+	[C(LL)] = {
+		[C(OP_READ)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+		[C(OP_WRITE)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+		[C(OP_PREFETCH)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+	},
+	[C(DTLB)] = {
+		[C(OP_READ)] = {
+			[C(RESULT_ACCESS)] =  RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] =  RISCV_OP_UNSUPP,
+		},
+		[C(OP_WRITE)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+		[C(OP_PREFETCH)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+	},
+	[C(ITLB)] = {
+		[C(OP_READ)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+		[C(OP_WRITE)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+		[C(OP_PREFETCH)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+	},
+	[C(BPU)] = {
+		[C(OP_READ)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+		[C(OP_WRITE)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+		[C(OP_PREFETCH)] = {
+			[C(RESULT_ACCESS)] = RISCV_OP_UNSUPP,
+			[C(RESULT_MISS)] = RISCV_OP_UNSUPP,
+		},
+	},
+};
+
+static int riscv_map_hw_event(u64 config)
+{
+	if (config >= riscv_pmu->max_events)
+		return -EINVAL;
+
+	return riscv_pmu->hw_events[config];
+}
+
+int riscv_map_cache_decode(u64 config, unsigned int *type,
+			   unsigned int *op, unsigned int *result)
+{
+	return -ENOENT;
+}
+
+static int riscv_map_cache_event(u64 config)
+{
+	unsigned int type, op, result;
+	int err = -ENOENT;
+		int code;
+
+	err = riscv_map_cache_decode(config, &type, &op, &result);
+	if (!riscv_pmu->cache_events || err)
+		return err;
+
+	if (type >= PERF_COUNT_HW_CACHE_MAX ||
+	    op >= PERF_COUNT_HW_CACHE_OP_MAX ||
+	    result >= PERF_COUNT_HW_CACHE_RESULT_MAX)
+		return -EINVAL;
+
+	code = (*riscv_pmu->cache_events)[type][op][result];
+	if (code == RISCV_OP_UNSUPP)
+		return -EINVAL;
+
+	return code;
+}
+
+/*
+ * Low-level functions: reading/writing counters
+ */
+
+static inline u64 read_counter(int idx)
+{
+	u64 val = 0;
+
+	switch (idx) {
+	case RISCV_PMU_CYCLE:
+		val = csr_read(cycle);
+		break;
+	case RISCV_PMU_INSTRET:
+		val = csr_read(instret);
+		break;
+	default:
+		WARN_ON_ONCE(idx < 0 ||	idx > RISCV_MAX_COUNTERS);
+		return -EINVAL;
+	}
+
+	return val;
+}
+
+static inline void write_counter(int idx, u64 value)
+{
+	/* currently not supported */
+	WARN_ON_ONCE(1);
+}
+
+/*
+ * pmu->read: read and update the counter
+ *
+ * Other architectures' implementation often have a xxx_perf_event_update
+ * routine, which can return counter values when called in the IRQ, but
+ * return void when being called by the pmu->read method.
+ */
+static void riscv_pmu_read(struct perf_event *event)
+{
+	struct hw_perf_event *hwc = &event->hw;
+	u64 prev_raw_count, new_raw_count;
+	u64 oldval;
+	int idx = hwc->idx;
+	u64 delta;
+
+	do {
+		prev_raw_count = local64_read(&hwc->prev_count);
+		new_raw_count = read_counter(idx);
+
+		oldval = local64_cmpxchg(&hwc->prev_count, prev_raw_count,
+					 new_raw_count);
+	} while (oldval != prev_raw_count);
+
+	/*
+	 * delta is the value to update the counter we maintain in the kernel.
+	 */
+	delta = (new_raw_count - prev_raw_count) &
+		((1ULL << riscv_pmu->counter_width) - 1);
+	local64_add(delta, &event->count);
+	/*
+	 * Something like local64_sub(delta, &hwc->period_left) here is
+	 * needed if there is an interrupt for perf.
+	 */
+}
+
+/*
+ * State transition functions:
+ *
+ * stop()/start() & add()/del()
+ */
+
+/*
+ * pmu->stop: stop the counter
+ */
+static void riscv_pmu_stop(struct perf_event *event, int flags)
+{
+	struct hw_perf_event *hwc = &event->hw;
+
+	WARN_ON_ONCE(hwc->state & PERF_HES_STOPPED);
+	hwc->state |= PERF_HES_STOPPED;
+
+	if ((flags & PERF_EF_UPDATE) && !(hwc->state & PERF_HES_UPTODATE)) {
+		riscv_pmu->pmu->read(event);
+		hwc->state |= PERF_HES_UPTODATE;
+	}
+}
+
+/*
+ * pmu->start: start the event.
+ */
+static void riscv_pmu_start(struct perf_event *event, int flags)
+{
+	struct hw_perf_event *hwc = &event->hw;
+
+	if (WARN_ON_ONCE(!(event->hw.state & PERF_HES_STOPPED)))
+		return;
+
+	if (flags & PERF_EF_RELOAD) {
+		WARN_ON_ONCE(!(event->hw.state & PERF_HES_UPTODATE));
+
+		/*
+		 * Set the counter to the period to the next interrupt here,
+		 * if you have any.
+		 */
+	}
+
+	hwc->state = 0;
+	perf_event_update_userpage(event);
+
+	/*
+	 * Since we cannot write to counters, this serves as an initialization
+	 * to the delta-mechanism in pmu->read(); otherwise, the delta would be
+	 * wrong when pmu->read is called for the first time.
+	 */
+	local64_set(&hwc->prev_count, read_counter(hwc->idx));
+}
+
+/*
+ * pmu->add: add the event to PMU.
+ */
+static int riscv_pmu_add(struct perf_event *event, int flags)
+{
+	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
+	struct hw_perf_event *hwc = &event->hw;
+
+	if (cpuc->n_events == riscv_pmu->num_counters)
+		return -ENOSPC;
+
+	/*
+	 * We don't have general conunters, so no binding-event-to-counter
+	 * process here.
+	 *
+	 * Indexing using hwc->config generally not works, since config may
+	 * contain extra information, but here the only info we have in
+	 * hwc->config is the event index.
+	 */
+	hwc->idx = hwc->config;
+	cpuc->events[hwc->idx] = event;
+	cpuc->n_events++;
+
+	hwc->state = PERF_HES_UPTODATE | PERF_HES_STOPPED;
+
+	if (flags & PERF_EF_START)
+		riscv_pmu->pmu->start(event, PERF_EF_RELOAD);
+
+	return 0;
+}
+
+/*
+ * pmu->del: delete the event from PMU.
+ */
+static void riscv_pmu_del(struct perf_event *event, int flags)
+{
+	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
+	struct hw_perf_event *hwc = &event->hw;
+
+	cpuc->events[hwc->idx] = NULL;
+	cpuc->n_events--;
+	riscv_pmu->pmu->stop(event, PERF_EF_UPDATE);
+	perf_event_update_userpage(event);
+}
+
+/*
+ * Interrupt: a skeletion for reference.
+ */
+
+static DEFINE_MUTEX(pmc_reserve_mutex);
+
+irqreturn_t riscv_base_pmu_handle_irq(int irq_num, void *dev)
+{
+	return IRQ_NONE;
+}
+
+static int reserve_pmc_hardware(void)
+{
+	int err = 0;
+
+	mutex_lock(&pmc_reserve_mutex);
+	if (riscv_pmu->irq >= 0 && riscv_pmu->handle_irq) {
+		err = request_irq(riscv_pmu->irq, riscv_pmu->handle_irq,
+				  IRQF_PERCPU, "riscv-base-perf", NULL);
+	}
+	mutex_unlock(&pmc_reserve_mutex);
+
+	return err;
+}
+
+void release_pmc_hardware(void)
+{
+	mutex_lock(&pmc_reserve_mutex);
+	if (riscv_pmu->irq >= 0)
+		free_irq(riscv_pmu->irq, NULL);
+	mutex_unlock(&pmc_reserve_mutex);
+}
+
+/*
+ * Event Initialization/Finalization
+ */
+
+static atomic_t riscv_active_events = ATOMIC_INIT(0);
+
+static void riscv_event_destroy(struct perf_event *event)
+{
+	if (atomic_dec_return(&riscv_active_events) == 0)
+		release_pmc_hardware();
+}
+
+static int riscv_event_init(struct perf_event *event)
+{
+	struct perf_event_attr *attr = &event->attr;
+	struct hw_perf_event *hwc = &event->hw;
+	int err;
+	int code;
+
+	if (atomic_inc_return(&riscv_active_events) == 1) {
+		err = reserve_pmc_hardware();
+
+		if (err) {
+			pr_warn("PMC hardware not available\n");
+			atomic_dec(&riscv_active_events);
+			return -EBUSY;
+		}
+	}
+
+	switch (event->attr.type) {
+	case PERF_TYPE_HARDWARE:
+		code = riscv_pmu->map_hw_event(attr->config);
+		break;
+	case PERF_TYPE_HW_CACHE:
+		code = riscv_pmu->map_cache_event(attr->config);
+		break;
+	case PERF_TYPE_RAW:
+		return -EOPNOTSUPP;
+	default:
+		return -ENOENT;
+	}
+
+	event->destroy = riscv_event_destroy;
+	if (code < 0) {
+		event->destroy(event);
+		return code;
+	}
+
+	/*
+	 * idx is set to -1 because the index of a general event should not be
+	 * decided until binding to some counter in pmu->add().
+	 *
+	 * But since we don't have such support, later in pmu->add(), we just
+	 * use hwc->config as the index instead.
+	 */
+	hwc->config = code;
+	hwc->idx = -1;
+
+	return 0;
+}
+
+/*
+ * Initialization
+ */
+
+static struct pmu min_pmu = {
+	.name		= "riscv-base",
+	.event_init	= riscv_event_init,
+	.add		= riscv_pmu_add,
+	.del		= riscv_pmu_del,
+	.start		= riscv_pmu_start,
+	.stop		= riscv_pmu_stop,
+	.read		= riscv_pmu_read,
+};
+
+static const struct riscv_pmu riscv_base_pmu = {
+	.pmu = &min_pmu,
+	.max_events = ARRAY_SIZE(riscv_hw_event_map),
+	.map_hw_event = riscv_map_hw_event,
+	.hw_events = riscv_hw_event_map,
+	.map_cache_event = riscv_map_cache_event,
+	.cache_events = &riscv_cache_event_map,
+	.counter_width = 63,
+	.num_counters = RISCV_BASE_COUNTERS + 0,
+	.handle_irq = &riscv_base_pmu_handle_irq,
+
+	/* This means this PMU has no IRQ. */
+	.irq = -1,
+};
+
+static const struct of_device_id riscv_pmu_of_ids[] = {
+	{.compatible = "riscv,base-pmu",	.data = &riscv_base_pmu},
+	{ /* sentinel value */ }
+};
+
+int __init init_hw_perf_events(void)
+{
+	struct device_node *node = of_find_node_by_type(NULL, "pmu");
+	const struct of_device_id *of_id;
+
+	riscv_pmu = &riscv_base_pmu;
+
+	if (node) {
+		of_id = of_match_node(riscv_pmu_of_ids, node);
+
+		if (of_id)
+			riscv_pmu = of_id->data;
+	}
+
+	perf_pmu_register(riscv_pmu->pmu, "cpu", PERF_TYPE_RAW);
+	return 0;
+}
+arch_initcall(init_hw_perf_events);