linux-kernel - Re: [PATCH v2 1/5] perf cs-etm: Refactor instruction size handling

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Message-ID: <2b675e24-8b06-fbd6-ab73-214a6afb2a07@arm.com>
Date:   Mon, 23 Sep 2019 17:51:04 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Leo Yan <leo.yan@...aro.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Mike Leach <mike.leach@...aro.org>,
        Coresight ML <coresight@...ts.linaro.org>
Subject: Re: [PATCH v2 1/5] perf cs-etm: Refactor instruction size handling

Hi Leo,

On 23/09/2019 17:07, Leo Yan wrote:
> In cs-etm.c there have several functions need to know instruction size
> based on address, e.g. cs_etm__instr_addr() and cs_etm__copy_insn()
> these two functions both calculate the instruction size separately.
> Furthermore, if we consider to add new features later which also might
> require to calculate instruction size.
> 
> For this reason, this patch refactors the code to introduce a new
> function cs_etm__instr_size(), it will be a central place to calculate
> the instruction size based on ISA type and instruction address.
> 
> For a neat implementation, cs_etm__instr_addr() will always execute the
> loop without checking ISA type, this allows cs_etm__instr_size() and
> cs_etm__instr_addr() have no any duplicate code with each other and both
> functions can be changed independently later without breaking anything.
> As a side effect, cs_etm__instr_addr() will do a few more iterations for
> A32/A64 instructions, this would be fine if consider perf tool runs in
> the user space.
> 
> Signed-off-by: Leo Yan <leo.yan@...aro.org>

Your changes look fine to me. However, please see my comment below.

> ---
>   tools/perf/util/cs-etm.c | 48 +++++++++++++++++++++++-----------------
>   1 file changed, 28 insertions(+), 20 deletions(-)
> 
> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> index f87b9c1c9f9a..1de3f9361193 100644
> --- a/tools/perf/util/cs-etm.c
> +++ b/tools/perf/util/cs-etm.c
> @@ -917,6 +917,26 @@ static inline int cs_etm__t32_instr_size(struct cs_etm_queue *etmq,
>   	return ((instrBytes[1] & 0xF8) >= 0xE8) ? 4 : 2;
>   }
>   
> +static inline int cs_etm__instr_size(struct cs_etm_queue *etmq,
> +				     u8 trace_chan_id,
> +				     enum cs_etm_isa isa,
> +				     u64 addr)
> +{
> +	int insn_len;
> +
> +	/*
> +	 * T32 instruction size might be 32-bit or 16-bit, decide by calling
> +	 * cs_etm__t32_instr_size().
> +	 */
> +	if (isa == CS_ETM_ISA_T32)
> +		insn_len = cs_etm__t32_instr_size(etmq, trace_chan_id, addr);
> +	/* Otherwise, A64 and A32 instruction size are always 32-bit. */
> +	else
> +		insn_len = 4;
> +
> +	return insn_len;
> +}
> +
>   static inline u64 cs_etm__first_executed_instr(struct cs_etm_packet *packet)
>   {
>   	/* Returns 0 for the CS_ETM_DISCONTINUITY packet */
> @@ -941,19 +961,15 @@ static inline u64 cs_etm__instr_addr(struct cs_etm_queue *etmq,
>   				     const struct cs_etm_packet *packet,
>   				     u64 offset)
>   {
> -	if (packet->isa == CS_ETM_ISA_T32) {
> -		u64 addr = packet->start_addr;
> +	u64 addr = packet->start_addr;
>   
> -		while (offset > 0) {
> -			addr += cs_etm__t32_instr_size(etmq,
> -						       trace_chan_id, addr);
> -			offset--;
> -		}
> -		return addr;
> +	while (offset > 0) {

Given that offset is u64, the check above is not appropriate. You could either
change it to :
	while (offset) // if you are sure (s64)offset always is a postive
integer and we always reduce it by 1.

Otherwise you may switch the offset to a signed type. I understand that this
is not introduced by your changes. But you may fix that up in a separate patch.


Kind regards
Suzuki