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

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Message-ID: <Z1K6IckahmlME6Py@gmail.com>
Date: Fri, 6 Dec 2024 09:47:29 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Ravi Bangoria <ravi.bangoria@....com>
Cc: peterz@...radead.org, mingo@...hat.com, namhyung@...nel.org,
	acme@...nel.org, eranian@...gle.com, mark.rutland@....com,
	alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
	irogers@...gle.com, adrian.hunter@...el.com,
	kan.liang@...ux.intel.com, tglx@...utronix.de, bp@...en8.de,
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	santosh.shukla@....com, ananth.narayan@....com,
	sandipan.das@....com
Subject: Re: [PATCH v2 05/10] perf/amd/ibs: Don't allow freq mode event
 creation through ->config interface


* Ravi Bangoria <ravi.bangoria@....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