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Subject[PATCH 5/6] arm64: perf: Enable pmu counter direct access for perf event on armv8
Date
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: everytime 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.

Signed-off-by: Raphael Gault <raphael.gault@arm.com>
---
arch/arm64/include/asm/mmu.h | 6 +++++
arch/arm64/include/asm/mmu_context.h | 2 ++
arch/arm64/include/asm/perf_event.h | 14 ++++++++++
drivers/perf/arm_pmu.c | 38 ++++++++++++++++++++++++++++
4 files changed, 60 insertions(+)

diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
index 67ef25d037ea..9de4cf0b17c7 100644
--- a/arch/arm64/include/asm/mmu.h
+++ b/arch/arm64/include/asm/mmu.h
@@ -29,6 +29,12 @@

typedef struct {
atomic64_t id;
+
+ /*
+ * non-zero if userspace have access to hardware
+ * counters directly.
+ */
+ atomic_t pmu_direct_access;
void *vdso;
unsigned long flags;
} mm_context_t;
diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
index 2da3e478fd8f..33494af613d8 100644
--- a/arch/arm64/include/asm/mmu_context.h
+++ b/arch/arm64/include/asm/mmu_context.h
@@ -32,6 +32,7 @@
#include <asm-generic/mm_hooks.h>
#include <asm/cputype.h>
#include <asm/pgtable.h>
+#include <asm/perf_event.h>
#include <asm/sysreg.h>
#include <asm/tlbflush.h>

@@ -235,6 +236,7 @@ static inline void __switch_mm(struct mm_struct *next)
}

check_and_switch_context(next, cpu);
+ 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 c593761ba61c..32a6d604bb3b 100644
--- a/arch/arm64/include/asm/perf_event.h
+++ b/arch/arm64/include/asm/perf_event.h
@@ -19,6 +19,7 @@

#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)
@@ -234,4 +235,17 @@ 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/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
index 3f4c2ec7ff89..45a64f942864 100644
--- a/drivers/perf/arm_pmu.c
+++ b/drivers/perf/arm_pmu.c
@@ -24,6 +24,7 @@
#include <linux/irqdesc.h>

#include <asm/irq_regs.h>
+#include <asm/mmu_context.h>

static DEFINE_PER_CPU(struct arm_pmu *, cpu_armpmu);
static DEFINE_PER_CPU(int, cpu_irq);
@@ -786,6 +787,41 @@ static int armpmu_event_idx(struct perf_event *event)
return event->hw.idx;
}

+static void refresh_pmuserenr(void *mm)
+{
+ perf_switch_user_access(mm);
+}
+
+static void armpmu_event_mapped(struct perf_event *event, struct mm_struct *mm)
+{
+ 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_sem
+ * for write. If this changes, we'll need a different solution.
+ */
+ lockdep_assert_held_exclusive(&mm->mmap_sem);
+
+ if (atomic_inc_return(&mm->context.pmu_direct_access) == 1)
+ on_each_cpu(refresh_pmuserenr, mm, 1);
+}
+
+static void armpmu_event_unmapped(struct perf_event *event, struct mm_struct *mm)
+{
+ if (!(event->hw.flags & ARMPMU_EL0_RD_CNTR))
+ return;
+
+ if (atomic_dec_and_test(&mm->context.pmu_direct_access))
+ on_each_cpu_mask(mm_cpumask(mm), refresh_pmuserenr, NULL, 1);
+}
+
static struct arm_pmu *__armpmu_alloc(gfp_t flags)
{
struct arm_pmu *pmu;
@@ -807,6 +843,8 @@ static struct arm_pmu *__armpmu_alloc(gfp_t flags)
.pmu_enable = armpmu_enable,
.pmu_disable = armpmu_disable,
.event_init = armpmu_event_init,
+ .event_mapped = armpmu_event_mapped,
+ .event_unmapped = armpmu_event_unmapped,
.add = armpmu_add,
.del = armpmu_del,
.start = armpmu_start,
--
2.17.1
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