alpha: implement HW performance events on the EV67 and later CPUs

This implements hardware performance events for the EV67 and later CPUs
within the Linux performance events subsystem.  Only using the performance
monitoring unit in HP/Compaq's so called "Aggregrate mode" is supported.

The code has been implemented in a manner that makes extension to other
older Alpha CPUs relatively straightforward should some mug wish to
indulge themselves.
Signed-off-by: Michael Cree <mcree@orcon.net.nz>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jay Estabrook <jay.estabrook@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/alpha/include/asm/perf_event.h

@@ -2,8 +2,14 @@
 #define __ASM_ALPHA_PERF_EVENT_H
 
 /* Alpha only supports software events through this interface. */
-static inline void set_perf_event_pending(void) { }
+extern void set_perf_event_pending(void);
 
 #define PERF_EVENT_INDEX_OFFSET 0
 
+#ifdef CONFIG_PERF_EVENTS
+extern void init_hw_perf_events(void);
+#else
+static inline void init_hw_perf_events(void)    { }
+#endif
+
 #endif /* __ASM_ALPHA_PERF_EVENT_H */

arch/alpha/kernel/Makefile

@@ -15,6 +15,7 @@ obj-$(CONFIG_SMP)	+= smp.o
 obj-$(CONFIG_PCI)	+= pci.o pci_iommu.o pci-sysfs.o
 obj-$(CONFIG_SRM_ENV)	+= srm_env.o
 obj-$(CONFIG_MODULES)	+= module.o
+obj-$(CONFIG_PERF_EVENTS) += perf_event.o
 
 ifdef CONFIG_ALPHA_GENERIC
 

arch/alpha/kernel/irq_alpha.c

@@ -10,6 +10,7 @@
 
 #include <asm/machvec.h>
 #include <asm/dma.h>
+#include <asm/perf_event.h>
 
 #include "proto.h"
 #include "irq_impl.h"
@@ -111,6 +112,8 @@ init_IRQ(void)
 	wrent(entInt, 0);
 
 	alpha_mv.init_irq();
+
+	init_hw_perf_events();
 }
 
 /*

arch/alpha/kernel/perf_event.c

+/*
+ * Hardware performance events for the Alpha.
+ *
+ * We implement HW counts on the EV67 and subsequent CPUs only.
+ *
+ * (C) 2010 Michael J. Cree
+ *
+ * Somewhat based on the Sparc code, and to a lesser extent the PowerPC and
+ * ARM code, which are copyright by their respective authors.
+ */
+
+#include <linux/perf_event.h>
+#include <linux/kprobes.h>
+#include <linux/kernel.h>
+#include <linux/kdebug.h>
+#include <linux/mutex.h>
+
+#include <asm/hwrpb.h>
+#include <asm/atomic.h>
+#include <asm/irq.h>
+#include <asm/irq_regs.h>
+#include <asm/pal.h>
+#include <asm/wrperfmon.h>
+#include <asm/hw_irq.h>
+
+
+/* The maximum number of PMCs on any Alpha CPU whatsoever. */
+#define MAX_HWEVENTS 3
+#define PMC_NO_INDEX -1
+
+/* For tracking PMCs and the hw events they monitor on each CPU. */
+struct cpu_hw_events {
+	int			enabled;
+	/* Number of events scheduled; also number entries valid in arrays below. */
+	int			n_events;
+	/* Number events added since last hw_perf_disable(). */
+	int			n_added;
+	/* Events currently scheduled. */
+	struct perf_event	*event[MAX_HWEVENTS];
+	/* Event type of each scheduled event. */
+	unsigned long		evtype[MAX_HWEVENTS];
+	/* Current index of each scheduled event; if not yet determined
+	 * contains PMC_NO_INDEX.
+	 */
+	int			current_idx[MAX_HWEVENTS];
+	/* The active PMCs' config for easy use with wrperfmon(). */
+	unsigned long		config;
+	/* The active counters' indices for easy use with wrperfmon(). */
+	unsigned long		idx_mask;
+};
+DEFINE_PER_CPU(struct cpu_hw_events, cpu_hw_events);
+
+
+
+/*
+ * A structure to hold the description of the PMCs available on a particular
+ * type of Alpha CPU.
+ */
+struct alpha_pmu_t {
+	/* Mapping of the perf system hw event types to indigenous event types */
+	const int *event_map;
+	/* The number of entries in the event_map */
+	int  max_events;
+	/* The number of PMCs on this Alpha */
+	int  num_pmcs;
+	/*
+	 * All PMC counters reside in the IBOX register PCTR.  This is the
+	 * LSB of the counter.
+	 */
+	int  pmc_count_shift[MAX_HWEVENTS];
+	/*
+	 * The mask that isolates the PMC bits when the LSB of the counter
+	 * is shifted to bit 0.
+	 */
+	unsigned long pmc_count_mask[MAX_HWEVENTS];
+	/* The maximum period the PMC can count. */
+	unsigned long pmc_max_period[MAX_HWEVENTS];
+	/*
+	 * The maximum value that may be written to the counter due to
+	 * hardware restrictions is pmc_max_period - pmc_left.
+	 */
+	long pmc_left[3];
+	 /* Subroutine for allocation of PMCs.  Enforces constraints. */
+	int (*check_constraints)(struct perf_event **, unsigned long *, int);
+};
+
+/*
+ * The Alpha CPU PMU description currently in operation.  This is set during
+ * the boot process to the specific CPU of the machine.
+ */
+static const struct alpha_pmu_t *alpha_pmu;
+
+
+#define HW_OP_UNSUPPORTED -1
+
+/*
+ * The hardware description of the EV67, EV68, EV69, EV7 and EV79 PMUs
+ * follow. Since they are identical we refer to them collectively as the
+ * EV67 henceforth.
+ */
+
+/*
+ * EV67 PMC event types
+ *
+ * There is no one-to-one mapping of the possible hw event types to the
+ * actual codes that are used to program the PMCs hence we introduce our
+ * own hw event type identifiers.
+ */
+enum ev67_pmc_event_type {
+	EV67_CYCLES = 1,
+	EV67_INSTRUCTIONS,
+	EV67_BCACHEMISS,
+	EV67_MBOXREPLAY,
+	EV67_LAST_ET
+};
+#define EV67_NUM_EVENT_TYPES (EV67_LAST_ET-EV67_CYCLES)
+
+
+/* Mapping of the hw event types to the perf tool interface */
+static const int ev67_perfmon_event_map[] = {
+	[PERF_COUNT_HW_CPU_CYCLES]	 = EV67_CYCLES,
+	[PERF_COUNT_HW_INSTRUCTIONS]	 = EV67_INSTRUCTIONS,
+	[PERF_COUNT_HW_CACHE_REFERENCES] = HW_OP_UNSUPPORTED,
+	[PERF_COUNT_HW_CACHE_MISSES]	 = EV67_BCACHEMISS,
+};
+
+struct ev67_mapping_t {
+	int config;
+	int idx;
+};
+
+/*
+ * The mapping used for one event only - these must be in same order as enum
+ * ev67_pmc_event_type definition.
+ */
+static const struct ev67_mapping_t ev67_mapping[] = {
+	{EV67_PCTR_INSTR_CYCLES, 1},	 /* EV67_CYCLES, */
+	{EV67_PCTR_INSTR_CYCLES, 0},	 /* EV67_INSTRUCTIONS */
+	{EV67_PCTR_INSTR_BCACHEMISS, 1}, /* EV67_BCACHEMISS */
+	{EV67_PCTR_CYCLES_MBOX, 1}	 /* EV67_MBOXREPLAY */
+};
+
+
+/*
+ * Check that a group of events can be simultaneously scheduled on to the
+ * EV67 PMU.  Also allocate counter indices and config.
+ */
+static int ev67_check_constraints(struct perf_event **event,
+				unsigned long *evtype, int n_ev)
+{
+	int idx0;
+	unsigned long config;
+
+	idx0 = ev67_mapping[evtype[0]-1].idx;
+	config = ev67_mapping[evtype[0]-1].config;
+	if (n_ev == 1)
+		goto success;
+
+	BUG_ON(n_ev != 2);
+
+	if (evtype[0] == EV67_MBOXREPLAY || evtype[1] == EV67_MBOXREPLAY) {
+		/* MBOX replay traps must be on PMC 1 */
+		idx0 = (evtype[0] == EV67_MBOXREPLAY) ? 1 : 0;
+		/* Only cycles can accompany MBOX replay traps */
+		if (evtype[idx0] == EV67_CYCLES) {
+			config = EV67_PCTR_CYCLES_MBOX;
+			goto success;
+		}
+	}
+
+	if (evtype[0] == EV67_BCACHEMISS || evtype[1] == EV67_BCACHEMISS) {
+		/* Bcache misses must be on PMC 1 */
+		idx0 = (evtype[0] == EV67_BCACHEMISS) ? 1 : 0;
+		/* Only instructions can accompany Bcache misses */
+		if (evtype[idx0] == EV67_INSTRUCTIONS) {
+			config = EV67_PCTR_INSTR_BCACHEMISS;
+			goto success;
+		}
+	}
+
+	if (evtype[0] == EV67_INSTRUCTIONS || evtype[1] == EV67_INSTRUCTIONS) {
+		/* Instructions must be on PMC 0 */
+		idx0 = (evtype[0] == EV67_INSTRUCTIONS) ? 0 : 1;
+		/* By this point only cycles can accompany instructions */
+		if (evtype[idx0^1] == EV67_CYCLES) {
+			config = EV67_PCTR_INSTR_CYCLES;
+			goto success;
+		}
+	}
+
+	/* Otherwise, darn it, there is a conflict.  */
+	return -1;
+
+success:
+	event[0]->hw.idx = idx0;
+	event[0]->hw.config_base = config;
+	if (n_ev == 2) {
+		event[1]->hw.idx = idx0 ^ 1;
+		event[1]->hw.config_base = config;
+	}
+	return 0;
+}
+
+
+static const struct alpha_pmu_t ev67_pmu = {
+	.event_map = ev67_perfmon_event_map,
+	.max_events = ARRAY_SIZE(ev67_perfmon_event_map),
+	.num_pmcs = 2,
+	.pmc_count_shift = {EV67_PCTR_0_COUNT_SHIFT, EV67_PCTR_1_COUNT_SHIFT, 0},
+	.pmc_count_mask = {EV67_PCTR_0_COUNT_MASK,  EV67_PCTR_1_COUNT_MASK,  0},
+	.pmc_max_period = {(1UL<<20) - 1, (1UL<<20) - 1, 0},
+	.pmc_left = {16, 4, 0},
+	.check_constraints = ev67_check_constraints
+};
+
+
+
+/*
+ * Helper routines to ensure that we read/write only the correct PMC bits
+ * when calling the wrperfmon PALcall.
+ */
+static inline void alpha_write_pmc(int idx, unsigned long val)
+{
+	val &= alpha_pmu->pmc_count_mask[idx];
+	val <<= alpha_pmu->pmc_count_shift[idx];
+	val |= (1<<idx);
+	wrperfmon(PERFMON_CMD_WRITE, val);
+}
+
+static inline unsigned long alpha_read_pmc(int idx)
+{
+	unsigned long val;
+
+	val = wrperfmon(PERFMON_CMD_READ, 0);
+	val >>= alpha_pmu->pmc_count_shift[idx];
+	val &= alpha_pmu->pmc_count_mask[idx];
+	return val;
+}
+
+/* Set a new period to sample over */
+static int alpha_perf_event_set_period(struct perf_event *event,
+				struct hw_perf_event *hwc, int idx)
+{
+	long left = atomic64_read(&hwc->period_left);
+	long period = hwc->sample_period;
+	int ret = 0;
+
+	if (unlikely(left <= -period)) {
+		left = period;
+		atomic64_set(&hwc->period_left, left);
+		hwc->last_period = period;
+		ret = 1;
+	}
+
+	if (unlikely(left <= 0)) {
+		left += period;
+		atomic64_set(&hwc->period_left, left);
+		hwc->last_period = period;
+		ret = 1;
+	}
+
+	/*
+	 * Hardware restrictions require that the counters must not be
+	 * written with values that are too close to the maximum period.
+	 */
+	if (unlikely(left < alpha_pmu->pmc_left[idx]))
+		left = alpha_pmu->pmc_left[idx];
+
+	if (left > (long)alpha_pmu->pmc_max_period[idx])
+		left = alpha_pmu->pmc_max_period[idx];
+
+	atomic64_set(&hwc->prev_count, (unsigned long)(-left));
+
+	alpha_write_pmc(idx, (unsigned long)(-left));
+
+	perf_event_update_userpage(event);
+
+	return ret;
+}
+
+
+/*
+ * Calculates the count (the 'delta') since the last time the PMC was read.
+ *
+ * As the PMCs' full period can easily be exceeded within the perf system
+ * sampling period we cannot use any high order bits as a guard bit in the
+ * PMCs to detect overflow as is done by other architectures.  The code here
+ * calculates the delta on the basis that there is no overflow when ovf is
+ * zero.  The value passed via ovf by the interrupt handler corrects for
+ * overflow.
+ *
+ * This can be racey on rare occasions -- a call to this routine can occur
+ * with an overflowed counter just before the PMI service routine is called.
+ * The check for delta negative hopefully always rectifies this situation.
+ */
+static unsigned long alpha_perf_event_update(struct perf_event *event,
+					struct hw_perf_event *hwc, int idx, long ovf)
+{
+	long prev_raw_count, new_raw_count;
+	long delta;
+
+again:
+	prev_raw_count = atomic64_read(&hwc->prev_count);
+	new_raw_count = alpha_read_pmc(idx);
+
+	if (atomic64_cmpxchg(&hwc->prev_count, prev_raw_count,
+			     new_raw_count) != prev_raw_count)
+		goto again;
+
+	delta = (new_raw_count  - (prev_raw_count & alpha_pmu->pmc_count_mask[idx])) + ovf;
+
+	/* It is possible on very rare occasions that the PMC has overflowed
+	 * but the interrupt is yet to come.  Detect and fix this situation.
+	 */
+	if (unlikely(delta < 0)) {
+		delta += alpha_pmu->pmc_max_period[idx] + 1;
+	}
+
+	atomic64_add(delta, &event->count);
+	atomic64_sub(delta, &hwc->period_left);
+
+	return new_raw_count;
+}
+
+
+/*
+ * Collect all HW events into the array event[].
+ */
+static int collect_events(struct perf_event *group, int max_count,
+			  struct perf_event *event[], unsigned long *evtype,
+			  int *current_idx)
+{
+	struct perf_event *pe;
+	int n = 0;
+
+	if (!is_software_event(group)) {
+		if (n >= max_count)
+			return -1;
+		event[n] = group;
+		evtype[n] = group->hw.event_base;
+		current_idx[n++] = PMC_NO_INDEX;
+	}
+	list_for_each_entry(pe, &group->sibling_list, group_entry) {
+		if (!is_software_event(pe) && pe->state != PERF_EVENT_STATE_OFF) {
+			if (n >= max_count)
+				return -1;
+			event[n] = pe;
+			evtype[n] = pe->hw.event_base;
+			current_idx[n++] = PMC_NO_INDEX;
+		}
+	}
+	return n;
+}
+
+
+
+/*
+ * Check that a group of events can be simultaneously scheduled on to the PMU.
+ */
+static int alpha_check_constraints(struct perf_event **events,
+				   unsigned long *evtypes, int n_ev)
+{
+
+	/* No HW events is possible from hw_perf_group_sched_in(). */
+	if (n_ev == 0)
+		return 0;
+
+	if (n_ev > alpha_pmu->num_pmcs)
+		return -1;
+
+	return alpha_pmu->check_constraints(events, evtypes, n_ev);
+}
+
+
+/*
+ * If new events have been scheduled then update cpuc with the new
+ * configuration.  This may involve shifting cycle counts from one PMC to
+ * another.
+ */
+static void maybe_change_configuration(struct cpu_hw_events *cpuc)
+{
+	int j;
+
+	if (cpuc->n_added == 0)
+		return;
+
+	/* Find counters that are moving to another PMC and update */
+	for (j = 0; j < cpuc->n_events; j++) {
+		struct perf_event *pe = cpuc->event[j];
+
+		if (cpuc->current_idx[j] != PMC_NO_INDEX &&
+			cpuc->current_idx[j] != pe->hw.idx) {
+			alpha_perf_event_update(pe, &pe->hw, cpuc->current_idx[j], 0);
+			cpuc->current_idx[j] = PMC_NO_INDEX;
+		}
+	}
+
+	/* Assign to counters all unassigned events. */
+	cpuc->idx_mask = 0;
+	for (j = 0; j < cpuc->n_events; j++) {
+		struct perf_event *pe = cpuc->event[j];
+		struct hw_perf_event *hwc = &pe->hw;
+		int idx = hwc->idx;
+
+		if (cpuc->current_idx[j] != PMC_NO_INDEX) {
+			cpuc->idx_mask |= (1<<cpuc->current_idx[j]);
+			continue;
+		}
+
+		alpha_perf_event_set_period(pe, hwc, idx);
+		cpuc->current_idx[j] = idx;
+		cpuc->idx_mask |= (1<<cpuc->current_idx[j]);
+	}
+	cpuc->config = cpuc->event[0]->hw.config_base;
+}
+
+
+
+/* Schedule perf HW event on to PMU.
+ *  - this function is called from outside this module via the pmu struct
+ *    returned from perf event initialisation.
+ */
+static int alpha_pmu_enable(struct perf_event *event)
+{
+	struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
+	int n0;
+	int ret;
+	unsigned long flags;
+
+	/*
+	 * The Sparc code has the IRQ disable first followed by the perf
+	 * disable, however this can lead to an overflowed counter with the
+	 * PMI disabled on rare occasions.  The alpha_perf_event_update()
+	 * routine should detect this situation by noting a negative delta,
+	 * nevertheless we disable the PMCs first to enable a potential
+	 * final PMI to occur before we disable interrupts.
+	 */
+	perf_disable();
+	local_irq_save(flags);
+
+	/* Default to error to be returned */
+	ret = -EAGAIN;
+
+	/* Insert event on to PMU and if successful modify ret to valid return */
+	n0 = cpuc->n_events;
+	if (n0 < alpha_pmu->num_pmcs) {
+		cpuc->event[n0] = event;
+		cpuc->evtype[n0] = event->hw.event_base;
+		cpuc->current_idx[n0] = PMC_NO_INDEX;
+
+		if (!alpha_check_constraints(cpuc->event, cpuc->evtype, n0+1)) {
+			cpuc->n_events++;
+			cpuc->n_added++;
+			ret = 0;
+		}
+	}
+
+	local_irq_restore(flags);
+	perf_enable();
+
+	return ret;
+}
+
+
+
+/* Disable performance monitoring unit
+ *  - this function is called from outside this module via the pmu struct
+ *    returned from perf event initialisation.
+ */
+static void alpha_pmu_disable(struct perf_event *event)
+{
+	struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
+	struct hw_perf_event *hwc = &event->hw;
+	unsigned long flags;
+	int j;
+
+	perf_disable();
+	local_irq_save(flags);
+
+	for (j = 0; j < cpuc->n_events; j++) {
+		if (event == cpuc->event[j]) {
+			int idx = cpuc->current_idx[j];
+
+			/* Shift remaining entries down into the existing
+			 * slot.
+			 */
+			while (++j < cpuc->n_events) {
+				cpuc->event[j - 1] = cpuc->event[j];
+				cpuc->evtype[j - 1] = cpuc->evtype[j];
+				cpuc->current_idx[j - 1] =
+					cpuc->current_idx[j];
+			}
+
+			/* Absorb the final count and turn off the event. */
+			alpha_perf_event_update(event, hwc, idx, 0);
+			perf_event_update_userpage(event);
+
+			cpuc->idx_mask &= ~(1UL<<idx);
+			cpuc->n_events--;
+			break;
+		}
+	}
+
+	local_irq_restore(flags);
+	perf_enable();
+}
+
+
+static void alpha_pmu_read(struct perf_event *event)
+{
+	struct hw_perf_event *hwc = &event->hw;
+
+	alpha_perf_event_update(event, hwc, hwc->idx, 0);
+}
+
+
+static void alpha_pmu_unthrottle(struct perf_event *event)
+{
+	struct hw_perf_event *hwc = &event->hw;
+	struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
+
+	cpuc->idx_mask |= 1UL<<hwc->idx;
+	wrperfmon(PERFMON_CMD_ENABLE, (1UL<<hwc->idx));
+}
+
+
+/*
+ * Check that CPU performance counters are supported.
+ * - currently support EV67 and later CPUs.
+ * - actually some later revisions of the EV6 have the same PMC model as the
+ *     EV67 but we don't do suffiently deep CPU detection to detect them.
+ *     Bad luck to the very few people who might have one, I guess.
+ */
+static int supported_cpu(void)
+{
+	struct percpu_struct *cpu;
+	unsigned long cputype;
+
+	/* Get cpu type from HW */
+	cpu = (struct percpu_struct *)((char *)hwrpb + hwrpb->processor_offset);
+	cputype = cpu->type & 0xffffffff;
+	/* Include all of EV67, EV68, EV7, EV79 and EV69 as supported. */
+	return (cputype >= EV67_CPU) && (cputype <= EV69_CPU);
+}
+
+
+
+static void hw_perf_event_destroy(struct perf_event *event)
+{
+	/* Nothing to be done! */
+	return;
+}
+
+
+
+static int __hw_perf_event_init(struct perf_event *event)
+{
+	struct perf_event_attr *attr = &event->attr;
+	struct hw_perf_event *hwc = &event->hw;
+	struct perf_event *evts[MAX_HWEVENTS];
+	unsigned long evtypes[MAX_HWEVENTS];
+	int idx_rubbish_bin[MAX_HWEVENTS];
+	int ev;
+	int n;
+
+	/* We only support a limited range of HARDWARE event types with one
+	 * only programmable via a RAW event type.
+	 */
+	if (attr->type == PERF_TYPE_HARDWARE) {
+		if (attr->config >= alpha_pmu->max_events)
+			return -EINVAL;
+		ev = alpha_pmu->event_map[attr->config];
+	} else if (attr->type == PERF_TYPE_HW_CACHE) {
+		return -EOPNOTSUPP;
+	} else if (attr->type == PERF_TYPE_RAW) {
+		ev = attr->config & 0xff;
+	} else {
+		return -EOPNOTSUPP;
+	}
+
+	if (ev < 0) {
+		return ev;
+	}
+
+	/* The EV67 does not support mode exclusion */
+	if (attr->exclude_kernel || attr->exclude_user
+			|| attr->exclude_hv || attr->exclude_idle) {
+		return -EPERM;
+	}
+
+	/*
+	 * We place the event type in event_base here and leave calculation
+	 * of the codes to programme the PMU for alpha_pmu_enable() because
+	 * it is only then we will know what HW events are actually
+	 * scheduled on to the PMU.  At that point the code to programme the
+	 * PMU is put into config_base and the PMC to use is placed into
+	 * idx.  We initialise idx (below) to PMC_NO_INDEX to indicate that
+	 * it is yet to be determined.
+	 */
+	hwc->event_base = ev;
+
+	/* Collect events in a group together suitable for calling
+	 * alpha_check_constraints() to verify that the group as a whole can
+	 * be scheduled on to the PMU.
+	 */
+	n = 0;
+	if (event->group_leader != event) {
+		n = collect_events(event->group_leader,
+				alpha_pmu->num_pmcs - 1,
+				evts, evtypes, idx_rubbish_bin);
+		if (n < 0)
+			return -EINVAL;
+	}
+	evtypes[n] = hwc->event_base;
+	evts[n] = event;
+
+	if (alpha_check_constraints(evts, evtypes, n + 1))
+		return -EINVAL;
+
+	/* Indicate that PMU config and idx are yet to be determined. */
+	hwc->config_base = 0;
+	hwc->idx = PMC_NO_INDEX;
+
+	event->destroy = hw_perf_event_destroy;
+
+	/*
+	 * Most architectures reserve the PMU for their use at this point.
+	 * As there is no existing mechanism to arbitrate usage and there
+	 * appears to be no other user of the Alpha PMU we just assume
+	 * that we can just use it, hence a NO-OP here.
+	 *
+	 * Maybe an alpha_reserve_pmu() routine should be implemented but is
+	 * anything else ever going to use it?
+	 */
+
+	if (!hwc->sample_period) {
+		hwc->sample_period = alpha_pmu->pmc_max_period[0];
+		hwc->last_period = hwc->sample_period;
+		atomic64_set(&hwc->period_left, hwc->sample_period);
+	}
+
+	return 0;
+}
+
+static const struct pmu pmu = {
+	.enable		= alpha_pmu_enable,
+	.disable	= alpha_pmu_disable,
+	.read		= alpha_pmu_read,
+	.unthrottle	= alpha_pmu_unthrottle,
+};
+
+
+/*
+ * Main entry point to initialise a HW performance event.
+ */
+const struct pmu *hw_perf_event_init(struct perf_event *event)
+{
+	int err;
+
+	if (!alpha_pmu)
+		return ERR_PTR(-ENODEV);
+
+	/* Do the real initialisation work. */
+	err = __hw_perf_event_init(event);
+
+	if (err)
+		return ERR_PTR(err);
+
+	return &pmu;
+}
+
+
+
+/*
+ * Main entry point - enable HW performance counters.
+ */
+void hw_perf_enable(void)
+{
+	struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
+
+	if (cpuc->enabled)
+		return;
+
+	cpuc->enabled = 1;
+	barrier();
+
+	if (cpuc->n_events > 0) {
+		/* Update cpuc with information from any new scheduled events. */
+		maybe_change_configuration(cpuc);
+
+		/* Start counting the desired events. */
+		wrperfmon(PERFMON_CMD_LOGGING_OPTIONS, EV67_PCTR_MODE_AGGREGATE);
+		wrperfmon(PERFMON_CMD_DESIRED_EVENTS, cpuc->config);
+		wrperfmon(PERFMON_CMD_ENABLE, cpuc->idx_mask);
+	}
+}
+
+
+/*
+ * Main entry point - disable HW performance counters.
+ */
+
+void hw_perf_disable(void)
+{
+	struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
+
+	if (!cpuc->enabled)
+		return;
+
+	cpuc->enabled = 0;
+	cpuc->n_added = 0;
+
+	wrperfmon(PERFMON_CMD_DISABLE, cpuc->idx_mask);
+}
+
+
+/*
+ * Main entry point - don't know when this is called but it
+ * obviously dumps debug info.
+ */
+void perf_event_print_debug(void)
+{
+	unsigned long flags;
+	unsigned long pcr;
+	int pcr0, pcr1;
+	int cpu;
+
+	if (!supported_cpu())
+		return;
+
+	local_irq_save(flags);
+
+	cpu = smp_processor_id();
+
+	pcr = wrperfmon(PERFMON_CMD_READ, 0);
+	pcr0 = (pcr >> alpha_pmu->pmc_count_shift[0]) & alpha_pmu->pmc_count_mask[0];
+	pcr1 = (pcr >> alpha_pmu->pmc_count_shift[1]) & alpha_pmu->pmc_count_mask[1];
+
+	pr_info("CPU#%d: PCTR0[%06x] PCTR1[%06x]\n", cpu, pcr0, pcr1);
+
+	local_irq_restore(flags);
+}
+
+
+/*
+ * Performance Monitoring Interrupt Service Routine called when a PMC
+ * overflows.  The PMC that overflowed is passed in la_ptr.
+ */
+static void alpha_perf_event_irq_handler(unsigned long la_ptr,
+					struct pt_regs *regs)
+{
+	struct cpu_hw_events *cpuc;
+	struct perf_sample_data data;
+	struct perf_event *event;
+	struct hw_perf_event *hwc;
+	int idx, j;
+
+	__get_cpu_var(irq_pmi_count)++;
+	cpuc = &__get_cpu_var(cpu_hw_events);
+
+	/* Completely counting through the PMC's period to trigger a new PMC
+	 * overflow interrupt while in this interrupt routine is utterly
+	 * disastrous!  The EV6 and EV67 counters are sufficiently large to
+	 * prevent this but to be really sure disable the PMCs.
+	 */
+	wrperfmon(PERFMON_CMD_DISABLE, cpuc->idx_mask);
+
+	/* la_ptr is the counter that overflowed. */
+	if (unlikely(la_ptr >= perf_max_events)) {
+		/* This should never occur! */
+		irq_err_count++;
+		pr_warning("PMI: silly index %ld\n", la_ptr);
+		wrperfmon(PERFMON_CMD_ENABLE, cpuc->idx_mask);
+		return;
+	}
+
+	idx = la_ptr;
+
+	perf_sample_data_init(&data, 0);
+	for (j = 0; j < cpuc->n_events; j++) {
+		if (cpuc->current_idx[j] == idx)
+			break;
+	}
+
+	if (unlikely(j == cpuc->n_events)) {
+		/* This can occur if the event is disabled right on a PMC overflow. */
+		wrperfmon(PERFMON_CMD_ENABLE, cpuc->idx_mask);
+		return;
+	}
+
+	event = cpuc->event[j];
+
+	if (unlikely(!event)) {
+		/* This should never occur! */
+		irq_err_count++;
+		pr_warning("PMI: No event at index %d!\n", idx);
+		wrperfmon(PERFMON_CMD_ENABLE, cpuc->idx_mask);
+		return;
+	}
+
+	hwc = &event->hw;
+	alpha_perf_event_update(event, hwc, idx, alpha_pmu->pmc_max_period[idx]+1);
+	data.period = event->hw.last_period;
+
+	if (alpha_perf_event_set_period(event, hwc, idx)) {
+		if (perf_event_overflow(event, 1, &data, regs)) {
+			/* Interrupts coming too quickly; "throttle" the
+			 * counter, i.e., disable it for a little while.
+			 */
+			cpuc->idx_mask &= ~(1UL<<idx);
+		}
+	}
+	wrperfmon(PERFMON_CMD_ENABLE, cpuc->idx_mask);
+
+	return;
+}
+
+
+
+/*
+ * Init call to initialise performance events at kernel startup.
+ */
+void __init init_hw_perf_events(void)
+{
+	pr_info("Performance events: ");
+
+	if (!supported_cpu()) {
+		pr_cont("No support for your CPU.\n");
+		return;
+	}
+
+	pr_cont("Supported CPU type!\n");
+
+	/* Override performance counter IRQ vector */
+
+	perf_irq = alpha_perf_event_irq_handler;
+
+	/* And set up PMU specification */
+	alpha_pmu = &ev67_pmu;
+	perf_max_events = alpha_pmu->num_pmcs;
+}
+

arch/alpha/kernel/time.c

@@ -41,6 +41,7 @@
 #include <linux/init.h>
 #include <linux/bcd.h>
 #include <linux/profile.h>
+#include <linux/perf_event.h>
 
 #include <asm/uaccess.h>
 #include <asm/io.h>
@@ -82,6 +83,26 @@ static struct {
 
 unsigned long est_cycle_freq;
 
+#ifdef CONFIG_PERF_EVENTS
+
+DEFINE_PER_CPU(u8, perf_event_pending);
+
+#define set_perf_event_pending_flag()  __get_cpu_var(perf_event_pending) = 1
+#define test_perf_event_pending()      __get_cpu_var(perf_event_pending)
+#define clear_perf_event_pending()     __get_cpu_var(perf_event_pending) = 0
+
+void set_perf_event_pending(void)
+{
+	set_perf_event_pending_flag();
+}
+
+#else  /* CONFIG_PERF_EVENTS */
+
+#define test_perf_event_pending()      0
+#define clear_perf_event_pending()
+
+#endif /* CONFIG_PERF_EVENTS */
+
 
 static inline __u32 rpcc(void)
 {
@@ -175,6 +196,11 @@ irqreturn_t timer_interrupt(int irq, void *dev)
 		update_process_times(user_mode(get_irq_regs()));
 #endif
 
+	if (test_perf_event_pending()) {
+		clear_perf_event_pending();
+		perf_event_do_pending();
+	}
+
 	return IRQ_HANDLED;
 }