[PATCH v6 02/10] arm64: perf: Enable PMU counter direct access for perf event

Rob Herring
2021-03-11T00:10:35Z

From: Raphael Gault <raphael.gault@arm.com>

Keep track of event opened with direct access to the hardware counters
and modify permissions while they are open.

The strategy used here is the same which x86 uses: every time an event
is mapped, the permissions are set if required. The atomic field added
in the mm_context helps keep track of the different event opened and
de-activate the permissions when all are unmapped.
We also need to update the permissions in the context switch code so
that tasks keep the right permissions.

In order to enable 64-bit counters for userspace when available, a new
config1 bit is added for userspace to indicate it wants userspace counter
access. This bit allows the kernel to decide if chaining should be
disabled and chaining and userspace access are incompatible.
The modes for config1 are as follows:

config1 = 0 or 2 : user access enabled and always 32-bit
config1 = 1 : user access disabled and always 64-bit (using chaining if needed)
config1 = 3 : user access enabled and counter size matches underlying counter.

User access is enabled with config1 == 0 so that we match x86 behavior
and don't need Arm specific code (outside the counter read).

Signed-off-by: Raphael Gault <raphael.gault@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
---
I'm not completely sure if using current->active_mm in an IPI is okay?
It seems to work in my testing.

Peter Z says (event->oncpu == smp_processor_id()) in the user page
update is always true, but my testing says otherwise[1].

v6:
- Add new attr.config1 rdpmc bit for userspace to hint it wants
userspace access when also requesting 64-bit counters.

v5:
- Only set cap_user_rdpmc if event is on current cpu
- Limit enabling/disabling access to CPUs associated with the PMU
(supported_cpus) and with the mm_struct matching current->active_mm.

v2:
- Move mapped/unmapped into arm64 code. Fixes arm32.
- Rebase on cap_user_time_short changes

Changes from Raphael's v4:
- Drop homogeneous check
- Disable access for chained counters
- Set pmc_width in user page

[1] https://lore.kernel.org/lkml/CAL_JsqK+eKef5NaVnBfARCjRE3MYhfBfe54F9YHKbsTnWqLmLw@mail.gmail.com/

user fix
---
arch/arm64/include/asm/mmu.h | 5 ++
arch/arm64/include/asm/mmu_context.h | 2 +
arch/arm64/include/asm/perf_event.h | 14 +++++
arch/arm64/kernel/perf_event.c | 86 +++++++++++++++++++++++++++-
4 files changed, 104 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
index 75beffe2ee8a..ee08447455da 100644
--- a/arch/arm64/include/asm/mmu.h
+++ b/arch/arm64/include/asm/mmu.h
@@ -18,6 +18,11 @@

typedef struct {
atomic64_t id;
+ /*
+ * non-zero if userspace have access to hardware
+ * counters directly.
+ */
+ atomic_t pmu_direct_access;
#ifdef CONFIG_COMPAT
void *sigpage;
#endif
diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
index 70ce8c1d2b07..ccb5ff417b42 100644
--- a/arch/arm64/include/asm/mmu_context.h
+++ b/arch/arm64/include/asm/mmu_context.h
@@ -21,6 +21,7 @@
#include <asm/proc-fns.h>
#include <asm-generic/mm_hooks.h>
#include <asm/cputype.h>
+#include <asm/perf_event.h>
#include <asm/sysreg.h>
#include <asm/tlbflush.h>

@@ -230,6 +231,7 @@ static inline void __switch_mm(struct mm_struct *next)
}

check_and_switch_context(next);
+ perf_switch_user_access(next);
}

static inline void
diff --git a/arch/arm64/include/asm/perf_event.h b/arch/arm64/include/asm/perf_event.h
index 60731f602d3e..112f3f63b79e 100644
--- a/arch/arm64/include/asm/perf_event.h
+++ b/arch/arm64/include/asm/perf_event.h
@@ -8,6 +8,7 @@

#include <asm/stack_pointer.h>
#include <asm/ptrace.h>
+#include <linux/mm_types.h>

#define ARMV8_PMU_MAX_COUNTERS 32
#define ARMV8_PMU_COUNTER_MASK (ARMV8_PMU_MAX_COUNTERS - 1)
@@ -254,4 +255,17 @@ extern unsigned long perf_misc_flags(struct pt_regs *regs);
(regs)->pstate = PSR_MODE_EL1h; \
}

+static inline void perf_switch_user_access(struct mm_struct *mm)
+{
+ if (!IS_ENABLED(CONFIG_PERF_EVENTS))
+ return;
+
+ if (atomic_read(&mm->context.pmu_direct_access)) {
+ write_sysreg(ARMV8_PMU_USERENR_ER|ARMV8_PMU_USERENR_CR,
+ pmuserenr_el0);
+ } else {
+ write_sysreg(0, pmuserenr_el0);
+ }
+}
+
#endif
diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
index 387838496955..9ad3cc523ef4 100644
--- a/arch/arm64/kernel/perf_event.c
+++ b/arch/arm64/kernel/perf_event.c
@@ -288,15 +288,22 @@ static const struct attribute_group armv8_pmuv3_events_attr_group = {

PMU_FORMAT_ATTR(event, "config:0-15");
PMU_FORMAT_ATTR(long, "config1:0");
+PMU_FORMAT_ATTR(rdpmc, "config1:1");

static inline bool armv8pmu_event_is_64bit(struct perf_event *event)
{
return event->attr.config1 & 0x1;
}

+static inline bool armv8pmu_event_want_user_access(struct perf_event *event)
+{
+ return event->attr.config1 & 0x2;
+}
+
static struct attribute *armv8_pmuv3_format_attrs[] = {
&format_attr_event.attr,
&format_attr_long.attr,
+ &format_attr_rdpmc.attr,
NULL,
};

@@ -356,6 +363,7 @@ static inline bool armv8pmu_event_is_chained(struct perf_event *event)
struct arm_pmu *cpu_pmu = to_arm_pmu(event->pmu);

return !WARN_ON(idx < 0) &&
+ !armv8pmu_event_want_user_access(event) &&
armv8pmu_event_is_64bit(event) &&
!armv8pmu_has_long_event(cpu_pmu) &&
(idx != ARMV8_IDX_CYCLE_COUNTER);
@@ -849,13 +857,17 @@ static int armv8pmu_get_event_idx(struct pmu_hw_events *cpuc,
if (evtype == ARMV8_PMUV3_PERFCTR_CPU_CYCLES) {
if (!test_and_set_bit(ARMV8_IDX_CYCLE_COUNTER, cpuc->used_mask))
return ARMV8_IDX_CYCLE_COUNTER;
+ else if (armv8pmu_event_is_64bit(event) &&
+ armv8pmu_event_want_user_access(event) &&
+ !armv8pmu_has_long_event(cpu_pmu))
+ return -EAGAIN;
}

/*
* Otherwise use events counters
*/
- if (armv8pmu_event_is_64bit(event) &&
- !armv8pmu_has_long_event(cpu_pmu))
+ if (armv8pmu_event_is_64bit(event) && !armv8pmu_has_long_event(cpu_pmu) &&
+ !armv8pmu_event_want_user_access(event))
return armv8pmu_get_chain_idx(cpuc, cpu_pmu);
else
return armv8pmu_get_single_idx(cpuc, cpu_pmu);
@@ -887,6 +899,46 @@ static int armv8pmu_access_event_idx(struct perf_event *event)
return event->hw.idx;
}

+static void refresh_pmuserenr(void *mm)
+{
+ if (mm == current->active_mm)
+ perf_switch_user_access(mm);
+}
+
+static void armv8pmu_event_mapped(struct perf_event *event, struct mm_struct *mm)
+{
+ struct arm_pmu *armpmu = to_arm_pmu(event->pmu);
+
+ if (!(event->hw.flags & ARMPMU_EL0_RD_CNTR))
+ return;
+
+ /*
+ * This function relies on not being called concurrently in two
+ * tasks in the same mm. Otherwise one task could observe
+ * pmu_direct_access > 1 and return all the way back to
+ * userspace with user access disabled while another task is still
+ * doing on_each_cpu_mask() to enable user access.
+ *
+ * For now, this can't happen because all callers hold mmap_lock
+ * for write. If this changes, we'll need a different solution.
+ */
+ lockdep_assert_held_write(&mm->mmap_lock);
+
+ if (atomic_inc_return(&mm->context.pmu_direct_access) == 1)
+ on_each_cpu_mask(&armpmu->supported_cpus, refresh_pmuserenr, mm, 1);
+}
+
+static void armv8pmu_event_unmapped(struct perf_event *event, struct mm_struct *mm)
+{
+ struct arm_pmu *armpmu = to_arm_pmu(event->pmu);
+
+ if (!(event->hw.flags & ARMPMU_EL0_RD_CNTR))
+ return;
+
+ if (atomic_dec_and_test(&mm->context.pmu_direct_access))
+ on_each_cpu_mask(&armpmu->supported_cpus, refresh_pmuserenr, mm, 1);
+}
+
/*
* Add an event filter to a given event.
*/
@@ -980,9 +1032,23 @@ static int __armv8_pmuv3_map_event(struct perf_event *event,
&armv8_pmuv3_perf_cache_map,
ARMV8_PMU_EVTYPE_EVENT);

- if (armv8pmu_event_is_64bit(event))
+ if (armv8pmu_event_want_user_access(event) || !armv8pmu_event_is_64bit(event)) {
+ event->hw.flags |= ARMPMU_EL0_RD_CNTR;
+ /*
+ * At this point, the counter is not assigned. If a 64-bit
+ * counter is requested, we must make sure the h/w has 64-bit
+ * counters if we set the event size to 64-bit because chaining
+ * is not supported with userspace access. This may still fail
+ * later on if the CPU cycle counter is in use.
+ */
+ if (armv8pmu_event_is_64bit(event) &&
+ (armv8pmu_has_long_event(armpmu) ||
+ hw_event_id == ARMV8_PMUV3_PERFCTR_CPU_CYCLES))
+ event->hw.flags |= ARMPMU_EVT_64BIT;
+ } else if (armv8pmu_event_is_64bit(event))
event->hw.flags |= ARMPMU_EVT_64BIT;

+
/* Only expose micro/arch events supported by this PMU */
if ((hw_event_id > 0) && (hw_event_id < ARMV8_PMUV3_MAX_COMMON_EVENTS)
&& test_bit(hw_event_id, armpmu->pmceid_bitmap)) {
@@ -1115,6 +1181,8 @@ static int armv8_pmu_init(struct arm_pmu *cpu_pmu, char *name,
cpu_pmu->filter_match = armv8pmu_filter_match;

cpu_pmu->pmu.event_idx = armv8pmu_access_event_idx;
+ cpu_pmu->pmu.event_mapped = armv8pmu_event_mapped;
+ cpu_pmu->pmu.event_unmapped = armv8pmu_event_unmapped;

cpu_pmu->name = name;
cpu_pmu->map_event = map_event;
@@ -1290,6 +1358,18 @@ void arch_perf_update_userpage(struct perf_event *event,
userpg->cap_user_time = 0;
userpg->cap_user_time_zero = 0;
userpg->cap_user_time_short = 0;
+ userpg->cap_user_rdpmc = !!(event->hw.flags & ARMPMU_EL0_RD_CNTR) &&
+ (event->oncpu == smp_processor_id());
+
+ if (userpg->cap_user_rdpmc) {
+ struct arm_pmu *cpu_pmu = to_arm_pmu(event->pmu);
+
+ if (armv8pmu_event_is_64bit(event) &&
+ (armv8pmu_has_long_event(cpu_pmu) || (userpg->index == 32)))
+ userpg->pmc_width = 64;
+ else
+ userpg->pmc_width = 32;
+ }

do {
rd = sched_clock_read_begin(&seq);
--
2.27.0