Re: [PATCH 4/7] perf: cs-etm: Validate options after applying themperf_pmu__format_bits

[Leo Yan]

On Fri, Apr 28, 2023 at 01:33:16PM +0100, James Clark wrote:

[...]

> > ETMv4 uses 'contextid' as well, since the user space needs to know which
> > exception level's PID should be traced, e.g. when CPU runs in EL2
> > 'contextid' is set as ETM_OPT_CTXTID2, the perf tool will set 'contextid2'
> > to tell driver to trace CONTEXTIDR_EL2.
> > 
> 
> That's still working because it reads the config term in the setup
> function rather than setting any one bit manually:
> 
>   if (!perf_cpu_map__empty(cpus)) {
>     evsel__set_config_if_unset(cs_etm_pmu, cs_etm_evsel,
>                                "timestamp", 1);
>     evsel__set_config_if_unset(cs_etm_pmu, cs_etm_evsel,
>                                "contextid", 1);
>   }

Okay, yeah, this change can set CTXTID or CTXTID2 bit if user doesn't
set anything.

> > We can only verify 'contextid', and set 'contextid1' or 'contextid2' based
> > on CPU running exception level, finally driver knows how to trace PID.
> > 
> > Thanks,
> > Leo
> > 
> 
> I'm not 100% sure what you mean by this. But previously the validation
> was looking at both contextid1 and contextid2 options and checking if
> either were supported if either were set.
> 
> I have the following change in mind, it fixes the backwards
> compatibility issue. And the validation should be exactly the same as it
> was before. Except for one bug that I found when both bits are requested
> which I've also fixed here:
> 
> From f1b9f56df29dfb4f2a7be25f009c79c86335587a Mon Sep 17 00:00:00 2001
> From: James Clark <james.clark@xxxxxxx>
> Date: Fri, 28 Apr 2023 10:29:52 +0100
> Subject: [PATCH] perf cs-etm: Fix contextid validation
> 
> Pre 5.11 kernels don't support 'contextid1' and 'contextid2' so
> validation would be skipped. By adding an additional check for
> 'contextid', old kernels will still have validation done even though
> contextid would either be contextid1 or contextid2.
> 
> Additionally now that it's possible to override options, an existing bug
> in the validation is revealed. 'val' is overwritten by the contextid1
> validation, and re-used for contextid2 validation causing it to always
> fail. '!val || val != 0x4' is the same as 'val != 0x4' because 0 is also
> != 4, so that expression can be simplified and the temp variable not
> overwritten.
> 
> Fixes: 35c51f83dd1e ("perf cs-etm: Validate options after applying them")
> Signed-off-by: James Clark <james.clark@xxxxxxx>

LGTM:

Reviewed-by: Leo Yan <leo.yan@xxxxxxxxxx

Thanks for the fixing!

> ---
>  tools/perf/arch/arm/util/cs-etm.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/perf/arch/arm/util/cs-etm.c b/tools/perf/arch/arm/util/cs-etm.c
> index 77cb03e6ff87..9ca040bfb1aa 100644
> --- a/tools/perf/arch/arm/util/cs-etm.c
> +++ b/tools/perf/arch/arm/util/cs-etm.c
> @@ -78,9 +78,9 @@ static int cs_etm_validate_context_id(struct auxtrace_record *itr,
>  	char path[PATH_MAX];
>  	int err;
>  	u32 val;
> -	u64 contextid =
> -		evsel->core.attr.config &
> -		(perf_pmu__format_bits(&cs_etm_pmu->format, "contextid1") |
> +	u64 contextid = evsel->core.attr.config &
> +		(perf_pmu__format_bits(&cs_etm_pmu->format, "contextid") |
> +		 perf_pmu__format_bits(&cs_etm_pmu->format, "contextid1") |
>  		 perf_pmu__format_bits(&cs_etm_pmu->format, "contextid2"));
>  
>  	if (!contextid)
> @@ -114,8 +114,7 @@ static int cs_etm_validate_context_id(struct auxtrace_record *itr,
>  		 *  0b00100 Maximum of 32-bit Context ID size.
>  		 *  All other values are reserved.
>  		 */
> -		val = BMVAL(val, 5, 9);
> -		if (!val || val != 0x4) {
> +		if (BMVAL(val, 5, 9) != 0x4) {
>  			pr_err("%s: CONTEXTIDR_EL1 isn't supported, disable with %s/contextid1=0/\n",
>  			       CORESIGHT_ETM_PMU_NAME, CORESIGHT_ETM_PMU_NAME);
>  			return -EINVAL;
> -- 
> 2.34.1