Re: [PATCH v3 02/10] perf/amd/ibs: Remove pointless sample period check

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Dec 10, 2024 at 09:34:41AM +0000, Ravi Bangoria wrote:
> Valid perf event sample period value for IBS PMUs (Fetch and Op both)
> is limited to multiple of 0x10. perf_ibs_init() has this check:
> 
>   if (!event->attr.sample_freq && hwc->sample_period & 0x0f)
>           return -EINVAL;
> 
> But it's broken since hwc->sample_period will always be 0 when
> event->attr.sample_freq is 0 (irrespective of event->attr.freq value.)
> 
> One option to fix this is to change the condition:
> 
>   - if (!event->attr.sample_freq && hwc->sample_period & 0x0f)
>   + if (!event->attr.freq && hwc->sample_period & 0x0f)
> 
> However, that will break all userspace tools which have been using IBS
> event with sample_period not multiple of 0x10.
> 
> Another option is to remove the condition altogether and mask lower
> nibble _silently_, same as what current code is inadvertently doing.
> I'm preferring this approach as it keeps the existing behavior.

Urgh... :/

> Acked-by: Namhyung Kim <namhyung@kernel.org>
> Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
> ---
>  arch/x86/events/amd/ibs.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
> index 4ca8006d2221..bd8919e7c3b1 100644
> --- a/arch/x86/events/amd/ibs.c
> +++ b/arch/x86/events/amd/ibs.c
> @@ -307,13 +307,8 @@ static int perf_ibs_init(struct perf_event *event)
>  		if (config & perf_ibs->cnt_mask)
>  			/* raw max_cnt may not be set */
>  			return -EINVAL;
> -		if (!event->attr.sample_freq && hwc->sample_period & 0x0f)
> -			/*
> -			 * lower 4 bits can not be set in ibs max cnt,
> -			 * but allowing it in case we adjust the
> -			 * sample period to set a frequency.
> -			 */
> -			return -EINVAL;
> +
> +		/* Silently mask off lower nibble. IBS hw mandates it. */
>  		hwc->sample_period &= ~0x0FULL;
>  		if (!hwc->sample_period)
>  			hwc->sample_period = 0x10;
> -- 
> 2.43.0
>