Re: [PATCH v11 4/5] arm64: perf: Enable PMU counter userspace access for perf event

[Mark Rutland]

On Tue, Oct 19, 2021 at 06:19:06PM -0500, Rob Herring wrote:
> Arm PMUs can support direct userspace access of counters which allows for
> low overhead (i.e. no syscall) self-monitoring of tasks. The same feature
> exists on x86 called 'rdpmc'. Unlike x86, userspace access will only be
> enabled for thread bound events. This could be extended if needed, but
> simplifies the implementation and reduces the chances for any
> information leaks (which the x86 implementation suffers from).
> 
> PMU EL0 access will be enabled when an event with userspace access is
> part of the thread's context. This includes when the event is not
> scheduled on the PMU. There's some additional overhead clearing
> dirty counters when access is enabled in order to prevent leaking
> disabled counter data from other tasks.
> 
> Unlike x86, enabling of userspace access must be requested with a new
> attr bit: config1:1. If the user requests userspace access with 64-bit
> counters, then the event open will fail if the h/w doesn't support
> 64-bit counters. Chaining is not supported with userspace access. The
> modes for config1 are as follows:
> 
> config1 = 0 : user access disabled and always 32-bit
> config1 = 1 : user access disabled and always 64-bit (using chaining if needed)
> config1 = 2 : user access enabled and always 32-bit
> config1 = 3 : user access enabled and always 64-bit
> 
> Based on work by Raphael Gault <raphael.gault@xxxxxxx>, but has been
> completely re-written.
> 
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-perf-users@xxxxxxxxxxxxxxx
> Signed-off-by: Rob Herring <robh@xxxxxxxxxx>
> ---
> v11:
>  - Add and use armv8pmu_event_has_user_read() helper
>  - s/armv8pmu_access_event_idx/armv8pmu_user_event_idx/
>  - Return error for user access when not a task bound event or no
>    64-bit counters when requested.
>  - Move custom sysctl handler function from prior patch to here
> 
> v10:
>  - Don't control enabling user access based on mmap(). Changing the
>    event_(un)mapped to run on the event's cpu doesn't work for x86.
>    Triggering on mmap() doesn't limit access in any way and complicates
>    the implementation.
>  - Drop dirty counter tracking and just clear all unused counters.
>  - Make the sysctl immediately disable access via IPI.
>  - Merge armv8pmu_event_is_chained() and armv8pmu_event_can_chain()
> 
> v9:
>  - Enabling/disabling of user access is now controlled in .start() and
>    mmap hooks which are now called on CPUs that the event is on.
>    Depends on rework of perf core and x86 RDPMC code posted here:
>    https://lore.kernel.org/lkml/20210728230230.1911468-1-robh@xxxxxxxxxx/
> 
> v8:
>  - Rework user access tracking and enabling to be done on task
>    context changes using sched_task() hook. This avoids the need for any
>    IPIs, mm_switch hooks or undef instr handler.
>  - Only support user access when explicitly requested on open and
>    only for a thread bound events. This avoids some of the information
>    leaks x86 has and simplifies the implementation.
> 
> v7:
>  - Clear disabled counters when user access is enabled for a task to
>    avoid leaking other tasks counter data.
>  - Rework context switch handling utilizing sched_task callback
>  - Add armv8pmu_event_can_chain() helper
>  - Rework config1 flags handling structure
>  - Use ARMV8_IDX_CYCLE_COUNTER_USER define for remapped user cycle
>    counter index
> 
> 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
> ---
>  arch/arm64/kernel/perf_event.c | 117 +++++++++++++++++++++++++++++++--
>  1 file changed, 110 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
> index 6ae20c4217af..c11f08e4eeb6 100644
> --- a/arch/arm64/kernel/perf_event.c
> +++ b/arch/arm64/kernel/perf_event.c
> @@ -285,6 +285,7 @@ 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 int sysctl_perf_user_access __read_mostly;
>  
> @@ -293,9 +294,15 @@ 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,
>  };
>  
> @@ -364,7 +371,7 @@ static const struct attribute_group armv8_pmuv3_caps_attr_group = {
>   */
>  #define	ARMV8_IDX_CYCLE_COUNTER	0
>  #define	ARMV8_IDX_COUNTER0	1
> -
> +#define	ARMV8_IDX_CYCLE_COUNTER_USER	32
>  
>  /*
>   * We unconditionally enable ARMv8.5-PMU long event counter support
> @@ -376,18 +383,22 @@ static bool armv8pmu_has_long_event(struct arm_pmu *cpu_pmu)
>  	return (cpu_pmu->pmuver >= ID_AA64DFR0_PMUVER_8_5);
>  }
>  
> +static inline bool armv8pmu_event_has_user_read(struct perf_event *event)
> +{
> +	return event->hw.flags & PERF_EVENT_FLAG_USER_READ_CNT;
> +}
> +
>  /*
>   * We must chain two programmable counters for 64 bit events,
>   * except when we have allocated the 64bit cycle counter (for CPU
> - * cycles event). This must be called only when the event has
> - * a counter allocated.
> + * cycles event) or when user space counter access is enabled.
>   */
>  static inline bool armv8pmu_event_is_chained(struct perf_event *event)
>  {
>  	int idx = event->hw.idx;
>  	struct arm_pmu *cpu_pmu = to_arm_pmu(event->pmu);
>  
> -	return !WARN_ON(idx < 0) &&
> +	return !armv8pmu_event_has_user_read(event) &&
>  	       armv8pmu_event_is_64bit(event) &&
>  	       !armv8pmu_has_long_event(cpu_pmu) &&
>  	       (idx != ARMV8_IDX_CYCLE_COUNTER);
> @@ -720,6 +731,27 @@ static inline u32 armv8pmu_getreset_flags(void)
>  	return value;
>  }
>  
> +static void armv8pmu_disable_user_access(void)
> +{
> +	write_sysreg(0, pmuserenr_el0);
> +}
> +
> +static void armv8pmu_enable_user_access(struct arm_pmu *cpu_pmu)
> +{
> +	int i;
> +	struct pmu_hw_events *cpuc = this_cpu_ptr(cpu_pmu->hw_events);
> +
> +	/* Clear any unused counters to avoid leaking their contents */
> +	for_each_clear_bit(i, cpuc->used_mask, cpu_pmu->num_events) {
> +		if (i == ARMV8_IDX_CYCLE_COUNTER)
> +			write_sysreg(0, pmccntr_el0);
> +		else
> +			armv8pmu_write_evcntr(i, 0);
> +	}
> +
> +	write_sysreg(ARMV8_PMU_USERENR_ER | ARMV8_PMU_USERENR_CR, pmuserenr_el0);
> +}
> +
>  static void armv8pmu_enable_event(struct perf_event *event)
>  {
>  	/*
> @@ -763,6 +795,14 @@ static void armv8pmu_disable_event(struct perf_event *event)
>  
>  static void armv8pmu_start(struct arm_pmu *cpu_pmu)
>  {
> +	struct perf_event_context *task_ctx =
> +		this_cpu_ptr(cpu_pmu->pmu.pmu_cpu_context)->task_ctx;
> +
> +	if (sysctl_perf_user_access && task_ctx && task_ctx->nr_user)
> +		armv8pmu_enable_user_access(cpu_pmu);
> +	else
> +		armv8pmu_disable_user_access();
> +
>  	/* Enable all counters */
>  	armv8pmu_pmcr_write(armv8pmu_pmcr_read() | ARMV8_PMU_PMCR_E);
>  }
> @@ -880,13 +920,16 @@ 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_chained(event))
>  		return	armv8pmu_get_chain_idx(cpuc, cpu_pmu);
>  	else
>  		return armv8pmu_get_single_idx(cpuc, cpu_pmu);
> @@ -902,6 +945,22 @@ static void armv8pmu_clear_event_idx(struct pmu_hw_events *cpuc,
>  		clear_bit(idx - 1, cpuc->used_mask);
>  }
>  
> +static int armv8pmu_user_event_idx(struct perf_event *event)
> +{
> +	if (!sysctl_perf_user_access || !armv8pmu_event_has_user_read(event))
> +		return 0;
> +
> +	/*
> +	 * We remap the cycle counter index to 32 to
> +	 * match the offset applied to the rest of
> +	 * the counter indices.
> +	 */
> +	if (event->hw.idx == ARMV8_IDX_CYCLE_COUNTER)
> +		return ARMV8_IDX_CYCLE_COUNTER_USER;
> +
> +	return event->hw.idx;
> +}
> +
>  /*
>   * Add an event filter to a given event.
>   */
> @@ -998,6 +1057,24 @@ static int __armv8_pmuv3_map_event(struct perf_event *event,
>  	if (armv8pmu_event_is_64bit(event))
>  		event->hw.flags |= ARMPMU_EVT_64BIT;
>  
> +	/*
> +	 * User events must be allocated into a single counter, and so
> +	 * must not be chained.
> +	 *
> +	 * Most 64-bit events require long counter support, but 64-bit
> +	 * CPU_CYCLES events can be placed into the dedicated cycle
> +	 * counter when this is free.
> +	 */
> +	if (armv8pmu_event_want_user_access(event)) {
> +		if ((armv8pmu_event_is_64bit(event) &&
> +		     (hw_event_id != ARMV8_PMUV3_PERFCTR_CPU_CYCLES) &&
> +		     !armv8pmu_has_long_event(armpmu)) ||
> +		    !(event->attach_state & PERF_ATTACH_TASK))
> +			return -EINVAL;
> +
> +		event->hw.flags |= PERF_EVENT_FLAG_USER_READ_CNT;
> +	}

Minor nit, but this'd be slightly more legibile as two conditions, and
thinking aboutthis some more, we should probably use -EOPNOTSUPP for the
unsupported counter size case, since that can vary by platform (and
aligns with what we do for mode exclusion).

So with this changed to:

	if (armv8pmu_event_want_user_access(event)) {
		if (!(event->attach_state & PERF_ATTACH_TASK))
			return -EINVAL;
		if (armv8pmu_event_is_64bit(event) &&
		    (hw_event_id != ARMV8_PMUV3_PERFCTR_CPU_CYCLES) &&
		    !armv8pmu_has_long_event(armpmu))
			return -EOPNOTSUPP;

		event->hw.flags |= PERF_EVENT_FLAG_USER_READ_CNT;
	}

Other than that (and modulo the PMSELR_EL0 issue noted in reply to the
cover letter), this looks good to me.

Thanks,
Mark.

> +
>  	/* 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)) {
> @@ -1106,13 +1183,29 @@ static int armv8pmu_probe_pmu(struct arm_pmu *cpu_pmu)
>  	return probe.present ? 0 : -ENODEV;
>  }
>  
> +static void armv8pmu_disable_user_access_ipi(void *unused)
> +{
> +	armv8pmu_disable_user_access();
> +}
> +
> +int armv8pmu_proc_user_access_handler(struct ctl_table *table, int write,
> +                void *buffer, size_t *lenp, loff_t *ppos)
> +{
> +	int ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> +	if (ret || !write || sysctl_perf_user_access)
> +		return ret;
> +
> +	on_each_cpu(armv8pmu_disable_user_access_ipi, NULL, 1);
> +	return 0;
> +}
> +
>  static struct ctl_table armv8_pmu_sysctl_table[] = {
>  	{
>  		.procname       = "perf_user_access",
>  		.data		= &sysctl_perf_user_access,
>  		.maxlen		= sizeof(unsigned int),
>  		.mode           = 0644,
> -		.proc_handler	= proc_dointvec_minmax,
> +		.proc_handler	= armv8pmu_proc_user_access_handler,
>  		.extra1		= SYSCTL_ZERO,
>  		.extra2		= SYSCTL_ONE,
>  	},
> @@ -1142,6 +1235,8 @@ static int armv8_pmu_init(struct arm_pmu *cpu_pmu, char *name,
>  	cpu_pmu->set_event_filter	= armv8pmu_set_event_filter;
>  	cpu_pmu->filter_match		= armv8pmu_filter_match;
>  
> +	cpu_pmu->pmu.event_idx		= armv8pmu_user_event_idx;
> +
>  	cpu_pmu->name			= name;
>  	cpu_pmu->map_event		= map_event;
>  	cpu_pmu->attr_groups[ARMPMU_ATTR_GROUP_EVENTS] = events ?
> @@ -1318,6 +1413,14 @@ 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 = armv8pmu_event_has_user_read(event);
> +
> +	if (userpg->cap_user_rdpmc) {
> +		if (event->hw.flags & ARMPMU_EVT_64BIT)
> +			userpg->pmc_width = 64;
> +		else
> +			userpg->pmc_width = 32;
> +	}
>  
>  	do {
>  		rd = sched_clock_read_begin(&seq);
> -- 
> 2.32.0
>