Subject: Re: [PATCH v6 8/8] perf cs-etm: Set sample flags for exception return packet

From: Mathieu Poirier <[email-masked]>
Date: Wed, 23 Jan 2019 14:51:14 -0700

On Sat, Jan 19, 2019 at 09:43:47AM +0800, Leo Yan wrote:
> When return from exception, we need to distinguish if it's system call
> return or for other type exceptions for setting sample flags.  Due to
> the exception return packet doesn't contain exception number, so we
> cannot decide sample flags based on exception number.
> 
> On the other hand, the exception return packet is followed by an
> instruction range packet; this range packet deliveries the start address
> after exception handling, we can check if it is a SVC instruction just
> before the start address.  If there has one SVC instruction is found
> ahead the return address, this means it's an exception return for system
> call; otherwise it is an normal return for other exceptions.
> 
> This patch is to set sample flags for exception return packet, firstly
> it simply set sample flags as PERF_IP_FLAG_INTERRUPT for all exception
> returns since at this point it doesn't know what's exactly the exception
> type.  We will defer to decide if it's an exception return for system
> call when the next instruction range packet comes, it checks if there
> has one SVC instruction prior to the start address and if so we will
> change sample flags to PERF_IP_FLAG_SYSCALLRET for system call
> return.
> 
> Signed-off-by: Leo Yan <[email-masked]>
> ---
>  tools/perf/util/cs-etm.c | 44 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 44 insertions(+)
> 
> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> index 052805de6513..7547a7178f46 100644
> --- a/tools/perf/util/cs-etm.c
> +++ b/tools/perf/util/cs-etm.c
> @@ -1372,6 +1372,20 @@ static int cs_etm__set_sample_flags(struct cs_etm_queue *etmq)
>  		if (prev_packet->sample_type == CS_ETM_DISCONTINUITY)
>  			prev_packet->flags |= PERF_IP_FLAG_BRANCH |
>  					      PERF_IP_FLAG_TRACE_BEGIN;
> +
> +		/*
> +		 * If the previous packet is an exception return packet
> +		 * and the return address just follows SVC instuction,
> +		 * it needs to calibrate the previous packet sample flags
> +		 * as PERF_IP_FLAG_SYSCALLRET.
> +		 */
> +		if (prev_packet->flags == (PERF_IP_FLAG_BRANCH |
> +					   PERF_IP_FLAG_RETURN |
> +					   PERF_IP_FLAG_INTERRUPT) &&

Would it make more sense to just look for prev-packet->sample_type ==
CS_ETM_EXCEPTION_RET ?

> +		    cs_etm__is_svc_instr(etmq, packet, packet->start_addr))
> +			prev_packet->flags = PERF_IP_FLAG_BRANCH |
> +					     PERF_IP_FLAG_RETURN |
> +					     PERF_IP_FLAG_SYSCALLRET;
>  		break;
>  	case CS_ETM_DISCONTINUITY:
>  		/*
> @@ -1422,6 +1436,36 @@ static int cs_etm__set_sample_flags(struct cs_etm_queue *etmq)
>  			prev_packet->flags = packet->flags;
>  		break;
>  	case CS_ETM_EXCEPTION_RET:
> +		/*
> +		 * When the exception return packet is inserted, since
> +		 * exception return packet is not used standalone for
> +		 * generating samples and it's affiliation to the previous
> +		 * instruction range packet; so set previous range packet
> +		 * flags to tell perf it is an exception return branch.
> +		 *
> +		 * The exception return can be for either system call or
> +		 * other exception types; unfortunately the packet doesn't
> +		 * contain exception type related info so we cannot decide
> +		 * the exception type purely based on exception return packet.
> +		 * If we record the exception number from exception packet and
> +		 * reuse it for excpetion return packet, this is not reliable
> +		 * due the trace can be discontinuity or the interrupt can
> +		 * be nested, thus the recorded exception number cannot be
> +		 * used for exception return packet for these two cases.
> +		 *
> +		 * For exception return packet, we only need to distinguish the
> +		 * packet is for system call or for other types.  Thus the
> +		 * decision can be deferred when receive the next packet which
> +		 * contains the return address, based on the return address we
> +		 * can read out the previous instruction and check if it's a
> +		 * system call instruction and then calibrate the sample flag
> +		 * as needed.
> +		 */
> +		if (prev_packet->sample_type == CS_ETM_RANGE)
> +			prev_packet->flags = PERF_IP_FLAG_BRANCH |
> +					     PERF_IP_FLAG_RETURN |
> +					     PERF_IP_FLAG_INTERRUPT;
> +		break;
>  	case CS_ETM_EMPTY:
>  	default:
>  		break;
> -- 
> 2.17.1
> 

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