Re: [PATCH v2 05/10] riscv: Prepare for user-space perf event mmap support

Provide all the necessary bits in the generic riscv pmu driver to be
able to mmap perf events in userspace: the heavy lifting lies in the
driver backend, namely the legacy and sbi implementations.

Note that arch_perf_update_userpage is almost a copy of arm64 code.

Signed-off-by: Alexandre Ghiti <redacted>
---
 arch/riscv/kernel/Makefile     |  2 +-
 arch/riscv/kernel/perf_event.c | 58 ++++++++++++++++++++++++++++++++++
 drivers/perf/riscv_pmu.c       | 41 ++++++++++++++++++++++++
 include/linux/perf/riscv_pmu.h |  4 +++
 4 files changed, 104 insertions(+), 1 deletion(-)
 create mode 100644 arch/riscv/kernel/perf_event.c

diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
index 4cf303a779ab..0d215fd9860d 100644
--- a/arch/riscv/kernel/Makefile
+++ b/arch/riscv/kernel/Makefile

@@ -70,7 +70,7 @@ obj-$(CONFIG_DYNAMIC_FTRACE)  += mcount-dyn.o

 obj-$(CONFIG_TRACE_IRQFLAGS)   += trace_irq.o

-obj-$(CONFIG_PERF_EVENTS)      += perf_callchain.o
+obj-$(CONFIG_PERF_EVENTS)      += perf_callchain.o perf_event.o
 obj-$(CONFIG_HAVE_PERF_REGS)   += perf_regs.o
 obj-$(CONFIG_RISCV_SBI)                += sbi.o
 ifeq ($(CONFIG_RISCV_SBI), y)

diff --git a/arch/riscv/kernel/perf_event.c b/arch/riscv/kernel/perf_event.c
new file mode 100644
index 000000000000..94174a0fc251
--- /dev/null
+++ b/arch/riscv/kernel/perf_event.c

@@ -0,0 +1,58 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <linux/sched_clock.h>
+
+void arch_perf_update_userpage(struct perf_event *event,
+                              struct perf_event_mmap_page *userpg, u64 now)
+{
+       struct clock_read_data *rd;
+       unsigned int seq;
+       u64 ns;
+
+       userpg->cap_user_time = 0;
+       userpg->cap_user_time_zero = 0;
+       userpg->cap_user_time_short = 0;
+       userpg->cap_user_rdpmc =
+               !!(event->hw.flags & PERF_EVENT_FLAG_USER_READ_CNT);
+
+       userpg->pmc_width = 64;
+

+       do {
+               rd = sched_clock_read_begin(&seq);
+
+               userpg->time_mult = rd->mult;
+               userpg->time_shift = rd->shift;
+               userpg->time_zero = rd->epoch_ns;
+               userpg->time_cycles = rd->epoch_cyc;
+               userpg->time_mask = rd->sched_clock_mask;
+
+               /*
+                * Subtract the cycle base, such that software that
+                * doesn't know about cap_user_time_short still 'works'
+                * assuming no wraps.
+                */
+               ns = mul_u64_u32_shr(rd->epoch_cyc, rd->mult, rd->shift);
+               userpg->time_zero -= ns;
+
+       } while (sched_clock_read_retry(seq));
+
+       userpg->time_offset = userpg->time_zero - now;
+
+       /*
+        * time_shift is not expected to be greater than 31 due to
+        * the original published conversion algorithm shifting a
+        * 32-bit value (now specifies a 64-bit value) - refer
+        * perf_event_mmap_page documentation in perf_event.h.
+        */
+       if (userpg->time_shift == 32) {
+               userpg->time_shift = 31;
+               userpg->time_mult >>= 1;
+       }
+
+       /*
+        * Internal timekeeping for enabled/running/stopped times
+        * is always computed with the sched_clock.
+        */
+       userpg->cap_user_time = 1;
+       userpg->cap_user_time_zero = 1;
+       userpg->cap_user_time_short = 1;
+}

diff --git a/drivers/perf/riscv_pmu.c b/drivers/perf/riscv_pmu.c
index ebca5eab9c9b..af69da268246 100644
--- a/drivers/perf/riscv_pmu.c
+++ b/drivers/perf/riscv_pmu.c

@@ -171,6 +171,8 @@ int riscv_pmu_event_set_period(struct perf_event *event)

        local64_set(&hwc->prev_count, (u64)-left);

+       perf_event_update_userpage(event);
+
        return overflow;
 }

@@ -267,6 +269,9 @@ static int riscv_pmu_event_init(struct perf_event *event)
        hwc->idx = -1;
        hwc->event_base = mapped_event;

+       if (rvpmu->event_init)
+               rvpmu->event_init(event);
+
        if (!is_sampling_event(event)) {
                /*
                 * For non-sampling runs, limit the sample_period to half

@@ -283,6 +288,39 @@ static int riscv_pmu_event_init(struct perf_event *event)
        return 0;
 }

+static int riscv_pmu_event_idx(struct perf_event *event)
+{
+       struct riscv_pmu *rvpmu = to_riscv_pmu(event->pmu);
+
+       if (!(event->hw.flags & PERF_EVENT_FLAG_USER_READ_CNT))
+               return 0;
+
+       if (rvpmu->csr_index)
+               return rvpmu->csr_index(event) + 1;
+
+       return 0;
+}
+
+static void riscv_pmu_event_mapped(struct perf_event *event, struct mm_struct *mm)
+{
+       struct riscv_pmu *rvpmu = to_riscv_pmu(event->pmu);
+
+       if (rvpmu->event_mapped) {
+               rvpmu->event_mapped(event, mm);
+               perf_event_update_userpage(event);
+       }
+}
+
+static void riscv_pmu_event_unmapped(struct perf_event *event, struct mm_struct *mm)
+{
+       struct riscv_pmu *rvpmu = to_riscv_pmu(event->pmu);
+
+       if (rvpmu->event_unmapped) {
+               rvpmu->event_unmapped(event, mm);
+               perf_event_update_userpage(event);
+       }
+}
+
 struct riscv_pmu *riscv_pmu_alloc(void)
 {
        struct riscv_pmu *pmu;

@@ -307,6 +345,9 @@ struct riscv_pmu *riscv_pmu_alloc(void)
        }
        pmu->pmu = (struct pmu) {
                .event_init     = riscv_pmu_event_init,
+               .event_mapped   = riscv_pmu_event_mapped,
+               .event_unmapped = riscv_pmu_event_unmapped,
+               .event_idx      = riscv_pmu_event_idx,
                .add            = riscv_pmu_add,
                .del            = riscv_pmu_del,
                .start          = riscv_pmu_start,

diff --git a/include/linux/perf/riscv_pmu.h b/include/linux/perf/riscv_pmu.h
index 9f70d94942e0..1452c8af3b67 100644
--- a/include/linux/perf/riscv_pmu.h
+++ b/include/linux/perf/riscv_pmu.h

@@ -55,6 +55,10 @@ struct riscv_pmu {
        void            (*ctr_start)(struct perf_event *event, u64 init_val);
        void            (*ctr_stop)(struct perf_event *event, unsigned long flag);
        int             (*event_map)(struct perf_event *event, u64 *config);
+       void            (*event_init)(struct perf_event *event);
+       void            (*event_mapped)(struct perf_event *event, struct mm_struct *mm);
+       void            (*event_unmapped)(struct perf_event *event, struct mm_struct *mm);
+       uint8_t         (*csr_index)(struct perf_event *event);

        struct cpu_hw_events    __percpu *hw_events;
        struct hlist_node       node;
--

2.37.2