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

Will Deacon
2021-03-30T15:33:21Z

On Wed, Mar 10, 2021 at 05:08:29PM -0700, Rob Herring wrote:
> 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.

In this last case, how does userspace know whether it got a 32-bit or a
64-bit 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].

Peter? Sounds like there's either a misunderstanding here or we have some
fundamental issue elsewhere.

> 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;

CONFIG_HW_PERF_EVENTS might be a better fit here.

> +
> + 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))

This logic is duplicated in armv8pmu_event_is_chained(); can you split it
into a helper, please?

> 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);
> +}

Why do we need to cross-call here? Seems like it would be a tonne simpler to
handle the trap. Is there a reason not to do that?

> +
> +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);

Given that the pmu_direct_access field is global per-mm, won't this go
wrong if multiple PMUs are opened by the same process but only a subset
are exposed to EL0? Perhaps pmu_direct_access should be treated as a mask
rather than a counter, so that we can 'and' it with the supported_cpus for
the PMU we're dealing with.

> +}
> +
> /*
> * 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;

Why do you set this for all 32-bit events? The logic here feels like it
could with a bit of untangling.

> + /*
> + * 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)))

The '32' here is the fake index for the cycle counter, right? I think that
was introduced in the previous patch, so let's add a #define for it.

Will