Re: [PATCH v4 6/7] arm64: perf: Enable pmu counter direct access for perf event on armv8
From: Jonathan Cameron
Date: Thu Aug 22 2019 - 12:40:10 EST
On Thu, 22 Aug 2019 15:42:19 +0100
Raphael Gault <raphael.gault@xxxxxxx> wrote:
> 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@xxxxxxx>
Hi Raphael,
One trivial comment inline.
Thanks,
Jonathan
> ---
> arch/arm64/include/asm/mmu.h | 6 ++++
> arch/arm64/include/asm/mmu_context.h | 2 ++
> arch/arm64/include/asm/perf_event.h | 14 ++++++++
> arch/arm64/kernel/perf_event.c | 1 +
> drivers/perf/arm_pmu.c | 54 ++++++++++++++++++++++++++++
> 5 files changed, 77 insertions(+)
>
> diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
> index fd6161336653..88ed4466bd06 100644
> --- a/arch/arm64/include/asm/mmu.h
> +++ b/arch/arm64/include/asm/mmu.h
> @@ -18,6 +18,12 @@
>
> 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 7ed0adb187a8..6e66ff940494 100644
> --- a/arch/arm64/include/asm/mmu_context.h
> +++ b/arch/arm64/include/asm/mmu_context.h
> @@ -21,6 +21,7 @@
> #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>
>
> @@ -224,6 +225,7 @@ 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 2bdbc79bbd01..ba58fa726631 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)
> @@ -223,4 +224,17 @@ 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 de9b001e8b7c..7de56f22d038 100644
> --- a/arch/arm64/kernel/perf_event.c
> +++ b/arch/arm64/kernel/perf_event.c
> @@ -1285,6 +1285,7 @@ void arch_perf_update_userpage(struct perf_event *event,
> */
> freq = arch_timer_get_rate();
> userpg->cap_user_time = 1;
> + userpg->cap_user_rdpmc = !!(event->hw.flags & ARMPMU_EL0_RD_CNTR);
>
> clocks_calc_mult_shift(&userpg->time_mult, &shift, freq,
> NSEC_PER_SEC, 0);
> diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
> index 2d06b8095a19..d0d3e523a4c4 100644
> --- a/drivers/perf/arm_pmu.c
> +++ b/drivers/perf/arm_pmu.c
> @@ -25,6 +25,7 @@
> #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);
> @@ -778,6 +779,57 @@ static void cpu_pmu_destroy(struct arm_pmu *cpu_pmu)
> &cpu_pmu->node);
> }
>
> +static void refresh_pmuserenr(void *mm)
> +{
> + perf_switch_user_access(mm);
> +}
> +
> +static int check_homogeneous_cap(struct perf_event *event, struct mm_struct *mm)
> +{
> + pr_info("checking HAS_HOMOGENEOUS_PMU");
Can we drop this spam from the good path. Makes a bit of a mess of my
terminal when running the test ;)
> + if (!cpus_have_cap(ARM64_HAS_HOMOGENEOUS_PMU)) {
> + pr_info("Disable direct access (!HAS_HOMOGENEOUS_PMU)");
> + atomic_set(&mm->context.pmu_direct_access, 0);
> + on_each_cpu(refresh_pmuserenr, mm, 1);
> + event->hw.flags &= ~ARMPMU_EL0_RD_CNTR;
> + return 0;
> + }
> +
> + return 1;
> +}
> +
> +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_write(&mm->mmap_sem);
> +
> + if (check_homogeneous_cap(event, mm) &&
> + 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 (check_homogeneous_cap(event, mm) &&
> + 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;
> @@ -799,6 +851,8 @@ 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,