Re: [PATCH 3/4] perf cs-etm: Track exception level

[James Clark <james.clark@arm.com>]

On 28/05/2023 12:05, Leo Yan wrote:
> On Wed, May 24, 2023 at 02:19:57PM +0100, James Clark wrote:
>> Currently we assume all trace belongs to the host machine so when
>> the decoder should be looking at the guest kernel maps it can crash
>> because it looks at the host ones instead.
>>
>> Avoid one scenario (guest kernel running at EL1) by assigning the
>> default guest machine to this trace. For userspace trace it's still not
>> possible to determine guest vs host, but the PIDs should help in this
>> case.
>>
>> Signed-off-by: James Clark <james.clark@arm.com>
>> ---
>>  .../perf/util/cs-etm-decoder/cs-etm-decoder.c |  7 +-
>>  tools/perf/util/cs-etm.c                      | 64 ++++++++++++++-----
>>  tools/perf/util/cs-etm.h                      |  5 +-
>>  3 files changed, 56 insertions(+), 20 deletions(-)
>>
>> diff --git a/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c b/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c
>> index 82a27ab90c8b..ac227cd03eb0 100644
>> --- a/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c
>> +++ b/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c
>> @@ -573,12 +573,13 @@ cs_etm_decoder__set_tid(struct cs_etm_queue *etmq,
>>  		break;
>>  	}
>>  
>> +	if (cs_etm__etmq_set_tid_el(etmq, tid, trace_chan_id,
>> +				 elem->context.exception_level))
>> +		return OCSD_RESP_FATAL_SYS_ERR;
>> +
>>  	if (tid == -1)
>>  		return OCSD_RESP_CONT;
>>  
>> -	if (cs_etm__etmq_set_tid(etmq, tid, trace_chan_id))
>> -		return OCSD_RESP_FATAL_SYS_ERR;
>> -
>>  	/*
>>  	 * A timestamp is generated after a PE_CONTEXT element so make sure
>>  	 * to rely on that coming one.
>> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
>> index a997fe79d458..b9ba19327f26 100644
>> --- a/tools/perf/util/cs-etm.c
>> +++ b/tools/perf/util/cs-etm.c
>> @@ -14,7 +14,6 @@
>>  #include <linux/types.h>
>>  #include <linux/zalloc.h>
>>  
>> -#include <opencsd/ocsd_if_types.h>
>>  #include <stdlib.h>
>>  
>>  #include "auxtrace.h"
>> @@ -87,6 +86,8 @@ struct cs_etm_traceid_queue {
>>  	union perf_event *event_buf;
>>  	struct thread *thread;
>>  	struct thread *prev_thread;
>> +	ocsd_ex_level prev_el;
>> +	ocsd_ex_level el;
>>  	struct branch_stack *last_branch;
>>  	struct branch_stack *last_branch_rb;
>>  	struct cs_etm_packet *prev_packet;
>> @@ -479,6 +480,7 @@ static int cs_etm__init_traceid_queue(struct cs_etm_queue *etmq,
>>  
>>  	queue = &etmq->etm->queues.queue_array[etmq->queue_nr];
>>  	tidq->trace_chan_id = trace_chan_id;
>> +	tidq->el = tidq->prev_el = ocsd_EL_unknown;
>>  	tidq->thread = machine__findnew_thread(&etm->session->machines.host, -1,
>>  					       queue->tid);
>>  	tidq->prev_thread = machine__idle_thread(&etm->session->machines.host);
>> @@ -618,6 +620,7 @@ static void cs_etm__packet_swap(struct cs_etm_auxtrace *etm,
>>  		tmp = tidq->packet;
>>  		tidq->packet = tidq->prev_packet;
>>  		tidq->prev_packet = tmp;
>> +		tidq->prev_el = tidq->el;
>>  		thread__put(tidq->prev_thread);
>>  		tidq->prev_thread = thread__get(tidq->thread);
>>  	}
>> @@ -879,11 +882,34 @@ static bool cs_etm__evsel_is_auxtrace(struct perf_session *session,
>>  	return evsel->core.attr.type == aux->pmu_type;
>>  }
>>  
>> -static u8 cs_etm__cpu_mode(struct cs_etm_queue *etmq, u64 address)
>> +static struct machine *cs_etm__get_machine(struct cs_etm_auxtrace *etm,
>> +					   ocsd_ex_level el)
>>  {
>> -	struct machine *machine;
>> +	/*
>> +	 * Not perfect, but assume anything in EL1 is the default guest, and
>> +	 * everything else is the host. nHVE and pKVM may not work with this
> 
> s/nHVE/nVHE ?
> 
> And it's good to say "if any virtulisation (e.g. pKVM) based on nVHE may
> not work with this ....".
> 
>> +	 * assumption. And distinguishing between guest and host userspaces
>> +	 * isn't currently supported either. Neither is multiple guest support.
>> +	 * All this does is reduce the likeliness of decode errors where we look
>> +	 * into the host kernel maps when it should have been the guest maps.
>> +	 */
>> +	switch (el) {
>> +	case ocsd_EL1:
>> +		return machines__find_guest(&etm->session->machines,
>> +					    DEFAULT_GUEST_KERNEL_ID);
> 
> I share the same concern with Mike that this will break perf for any
> Armv8 CPUs with nVHE (like Cortex-CA53, etc).
> 

I agree, I replied to Mike's comment with some thoughts.

> You could see CoreSight has trace data "elem->context.vmid", when a
> guest kernel runs in EL1, can we use "elem->context.vmid" as a guest
> kernel ID and finally retrieve the corresponding machine pointer?
> 

Do you mean something like this?

  static struct machine *cs_etm__get_machine(struct cs_etm_auxtrace *etm,
					   ocsd_ex_level el)
  {
    u64 pid_fmt;

    cs_etm__get_pid_fmt(trace_chan_id, &pid_fmt);

    if (pid_fmt == BIT(ETM_OPT_CTXTID))
      return return &etm->session->machines.host;

    switch (el) {
	case ocsd_EL1:
		return machines__find_guest(&etm->session->machines,
					    DEFAULT_GUEST_KERNEL_ID);
	case ocsd_EL3:
	case ocsd_EL2:
	case ocsd_EL0:
	case ocsd_EL_unknown:
	default:
		return &etm->session->machines.host;
	}
  }

I think this might work, maybe it's not as bad as I thought and we can
make it work out of the box.

> Thanks,
> Leo
> 
>> +	case ocsd_EL3:
>> +	case ocsd_EL2:
>> +	case ocsd_EL0:
>> +	case ocsd_EL_unknown:
>> +	default:
>> +		return &etm->session->machines.host;
>> +	}
>> +}
>>  
>> -	machine = &etmq->etm->session->machines.host;
>> +static u8 cs_etm__cpu_mode(struct cs_etm_queue *etmq, u64 address,
>> +			   ocsd_ex_level el)
>> +{
>> +	struct machine *machine = cs_etm__get_machine(etmq->etm, el);
>>  
>>  	if (address >= machine__kernel_start(machine)) {
>>  		if (machine__is_host(machine))
>> @@ -893,10 +919,14 @@ static u8 cs_etm__cpu_mode(struct cs_etm_queue *etmq, u64 address)
>>  	} else {
>>  		if (machine__is_host(machine))
>>  			return PERF_RECORD_MISC_USER;
>> -		else if (perf_guest)
>> +		else {
>> +			/*
>> +			 * Can't really happen at the moment because
>> +			 * cs_etm__get_machine() will always return
>> +			 * machines.host for any non EL1 trace.
>> +			 */
>>  			return PERF_RECORD_MISC_GUEST_USER;
>> -		else
>> -			return PERF_RECORD_MISC_HYPERVISOR;
>> +		}
>>  	}
>>  }
>>  
>> @@ -913,11 +943,12 @@ static u32 cs_etm__mem_access(struct cs_etm_queue *etmq, u8 trace_chan_id,
>>  	if (!etmq)
>>  		return 0;
>>  
>> -	cpumode = cs_etm__cpu_mode(etmq, address);
>>  	tidq = cs_etm__etmq_get_traceid_queue(etmq, trace_chan_id);
>>  	if (!tidq)
>>  		return 0;
>>  
>> +	cpumode = cs_etm__cpu_mode(etmq, address, tidq->el);
>> +
>>  	if (!thread__find_map(tidq->thread, cpumode, address, &al))
>>  		return 0;
>>  
>> @@ -1296,9 +1327,10 @@ cs_etm__get_trace(struct cs_etm_queue *etmq)
>>  }
>>  
>>  static void cs_etm__set_thread(struct cs_etm_auxtrace *etm,
>> -			       struct cs_etm_traceid_queue *tidq, pid_t tid)
>> +			       struct cs_etm_traceid_queue *tidq, pid_t tid,
>> +			       ocsd_ex_level el)
>>  {
>> -	struct machine *machine = &etm->session->machines.host;
>> +	struct machine *machine = cs_etm__get_machine(etm, el);
>>  
>>  	if (tid != -1) {
>>  		thread__zput(tidq->thread);
>> @@ -1308,10 +1340,12 @@ static void cs_etm__set_thread(struct cs_etm_auxtrace *etm,
>>  	/* Couldn't find a known thread */
>>  	if (!tidq->thread)
>>  		tidq->thread = machine__idle_thread(machine);
>> +
>> +	tidq->el = el;
>>  }
>>  
>> -int cs_etm__etmq_set_tid(struct cs_etm_queue *etmq,
>> -			 pid_t tid, u8 trace_chan_id)
>> +int cs_etm__etmq_set_tid_el(struct cs_etm_queue *etmq, pid_t tid, u8 trace_chan_id,
>> +			 ocsd_ex_level el)
>>  {
>>  	struct cs_etm_traceid_queue *tidq;
>>  
>> @@ -1319,7 +1353,7 @@ int cs_etm__etmq_set_tid(struct cs_etm_queue *etmq,
>>  	if (!tidq)
>>  		return -EINVAL;
>>  
>> -	cs_etm__set_thread(etmq->etm, tidq, tid);
>> +	cs_etm__set_thread(etmq->etm, tidq, tid, el);
>>  	return 0;
>>  }
>>  
>> @@ -1389,7 +1423,7 @@ static int cs_etm__synth_instruction_sample(struct cs_etm_queue *etmq,
>>  	struct perf_sample sample = {.ip = 0,};
>>  
>>  	event->sample.header.type = PERF_RECORD_SAMPLE;
>> -	event->sample.header.misc = cs_etm__cpu_mode(etmq, addr);
>> +	event->sample.header.misc = cs_etm__cpu_mode(etmq, addr, tidq->el);
>>  	event->sample.header.size = sizeof(struct perf_event_header);
>>  
>>  	/* Set time field based on etm auxtrace config. */
>> @@ -1448,7 +1482,7 @@ static int cs_etm__synth_branch_sample(struct cs_etm_queue *etmq,
>>  	ip = cs_etm__last_executed_instr(tidq->prev_packet);
>>  
>>  	event->sample.header.type = PERF_RECORD_SAMPLE;
>> -	event->sample.header.misc = cs_etm__cpu_mode(etmq, ip);
>> +	event->sample.header.misc = cs_etm__cpu_mode(etmq, ip, tidq->prev_el);
>>  	event->sample.header.size = sizeof(struct perf_event_header);
>>  
>>  	/* Set time field based on etm auxtrace config. */
>> diff --git a/tools/perf/util/cs-etm.h b/tools/perf/util/cs-etm.h
>> index 70cac0375b34..88e9b25a8a9f 100644
>> --- a/tools/perf/util/cs-etm.h
>> +++ b/tools/perf/util/cs-etm.h
>> @@ -232,10 +232,11 @@ int cs_etm__process_auxtrace_info(union perf_event *event,
>>  struct perf_event_attr *cs_etm_get_default_config(struct perf_pmu *pmu);
>>  
>>  #ifdef HAVE_CSTRACE_SUPPORT
>> +#include <opencsd/ocsd_if_types.h>
>>  int cs_etm__get_cpu(u8 trace_chan_id, int *cpu);
>>  int cs_etm__get_pid_fmt(u8 trace_chan_id, u64 *pid_fmt);
>> -int cs_etm__etmq_set_tid(struct cs_etm_queue *etmq,
>> -			 pid_t tid, u8 trace_chan_id);
>> +int cs_etm__etmq_set_tid_el(struct cs_etm_queue *etmq, pid_t tid, u8 trace_chan_id,
>> +			 ocsd_ex_level el);
>>  bool cs_etm__etmq_is_timeless(struct cs_etm_queue *etmq);
>>  void cs_etm__etmq_set_traceid_queue_timestamp(struct cs_etm_queue *etmq,
>>  					      u8 trace_chan_id);
>> -- 
>> 2.34.1
>>