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: <abe58150-735f-4d9d-8ff4-a20b0fc6b376@amd.com>
Date: Fri, 6 Dec 2024 14:37:04 +0530
From: Ravi Bangoria <ravi.bangoria@....com>
To: Ingo Molnar <mingo@...nel.org>
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,
 Ravi Bangoria <ravi.bangoria@....com>
Subject: Re: [PATCH v2 05/10] perf/amd/ibs: Don't allow freq mode event
 creation through ->config interface

Hi Ingo,

>> 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.

No. This patch does not prevent opening an IBS event in freq mode. User
can still open freq mode IBS event with usual interface attr->freq=1 and
attr->sample_freq=<freq>. i.e. 'perf top -F 1000' with IBS event will
work fine with this patch.

Thanks,
Ravi