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

[Atish Patra]

On Fri, May 12, 2023 at 1:58 AM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
>
> 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 <alexghiti@xxxxxxxxxxxx>
> ---
>  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;
> +

The counter width is 64 for cycle & instret. Other hpmcounter can have
different width.
This information should retrieved from counter info.

> +       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
>


-- 
Regards,
Atish