Re: [PATCH v2 1/9] perf: Add EVENT_ATTR_ID to simplify event attributes

[Will Deacon]

On Wed, May 19, 2021 at 05:51:51PM +0800, Qi Liu wrote:
> Similar EVENT_ATTR macros are defined in many PMU drivers,
> like HiSilicon PMU driver, Arm PMU driver, Arm SMMU PMU
> driver. So Add a generic macro to simplify code.
> 
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Signed-off-by: Qi Liu <liuqi115@xxxxxxxxxx>
> ---
>  include/linux/perf_event.h | 6 ++++++
>  kernel/events/core.c       | 2 ++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index f5a6a2f..d0aa74e 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1576,6 +1576,12 @@ static struct perf_pmu_events_attr _var = {				    \
>  	.event_str	= _str,						    \
>  };
>  
> +#define PMU_EVENT_ATTR_ID(_name, _id)					     \
> +	(&((struct perf_pmu_events_attr[]) {				     \
> +		{ .attr = __ATTR(_name, 0444, perf_event_sysfs_show, NULL),  \
> +		  .id = _id, }						     \
> +	})[0].attr.attr)
> +
>  #define PMU_FORMAT_ATTR(_name, _format)					\
>  static ssize_t								\
>  _name##_show(struct device *dev,					\
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 0ac818b..330d9cc 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -13295,6 +13295,8 @@ ssize_t perf_event_sysfs_show(struct device *dev, struct device_attribute *attr,
>  
>  	if (pmu_attr->event_str)
>  		return sprintf(page, "%s\n", pmu_attr->event_str);
> +	else
> +		return sprintf(page, "config=%#llx\n", pmu_attr->id);

I think it's a really bad idea to hardcode this here. For example, I think
this patch series breaks user ABI for the SMMU PMU which used to print:

	"event=0x%02llx\n"

and by the looks of it many of the other conversions are unsound too.

I'm all for a common macro, but the string needs to be determined by the
driver.

Will