Re: [PATCH v2 05/10] perf/amd/ibs: Don't allow freq mode event creation through ->config interface

[Ingo Molnar <mingo@kernel.org>]

* Ravi Bangoria <ravi.bangoria@amd.com> wrote:

> Most perf_event_attr->config bits directly maps to IBS_{FETCH|OP}_CTL
> MSR. Since the sample period is programmed in these control registers,
> IBS PMU driver allows opening an IBS event by setting sample period
> value directly in perf_event_attr->config instead of using explicit
> perf_event_attr->sample_period interface.
> 
> However, this logic is not applicable for freq mode events since the
> semantics of control register fields are applicable only to fixed
> sample period whereas the freq mode event adjusts sample period after
> each and every sample. Currently, IBS driver (unintentionally) allows
> creating freq mode event via ->config interface, which is semantically
> wrong as well as detrimental because it can be misused to bypass
> perf_event_max_sample_rate checks.

Then let's fix those rate checks?

AFAICS this patch limits functionality because the IBS driver would 
have to be fixed/enhanced to support frequency based events?

I'd strongly favor fixing/enhancing the driver instead, as 'perf top -F 
1000' is easy to use and it is a useful concept.

Thanks,

	Ingo