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From:   "Dey, Megha" <megha.dey@intel.com>
To:     Jiri Olsa <jolsa@redhat.com>, Megha Dey <megha.dey@linux.intel.com>
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Subject: RE: [PATCH V0 2/3] perf/x86/intel/bm.c: Add Intel Branch Monitoring
 support
Thread-Topic: [PATCH V0 2/3] perf/x86/intel/bm.c: Add Intel Branch
 Monitoring support
Thread-Index: AQHTVMuJ78tDWbmRNES2UyAi9yp0nKMErLeAgAm3zPA=
Date:   Sat, 11 Nov 2017 00:53:33 +0000
Message-ID: <C440BA31B54DCD4AAC682D2365C8FEE727FF0AB2@ORSMSX111.amr.corp.intel.com>
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 <20171104132611.GE8454@krava>
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>-----Original Message-----
>From: Jiri Olsa [mailto:jolsa@redhat.com]
>Sent: Saturday, November 4, 2017 6:26 AM
>To: Megha Dey <megha.dey@linux.intel.com>
>Cc: x86@kernel.org; linux-kernel@vger.kernel.org; linux-
>doc@vger.kernel.org; tglx@linutronix.de; mingo@redhat.com;
>hpa@zytor.com; andriy.shevchenko@linux.intel.com;
>kstewart@linuxfoundation.org; Yu, Yu-cheng <yu-cheng.yu@intel.com>;
>Brown, Len <len.brown@intel.com>; gregkh@linuxfoundation.org;
>peterz@infradead.org; acme@kernel.org;
>alexander.shishkin@linux.intel.com; namhyung@kernel.org;
>vikas.shivappa@linux.intel.com; pombredanne@nexb.com;
>me@kylehuey.com; bp@suse.de; Andrejczuk, Grzegorz
><grzegorz.andrejczuk@intel.com>; Luck, Tony <tony.luck@intel.com>;
>corbet@lwn.net; Shankar, Ravi V <ravi.v.shankar@intel.com>; Dey, Megha
><megha.dey@intel.com>
>Subject: Re: [PATCH V0 2/3] perf/x86/intel/bm.c: Add Intel Branch
>Monitoring support
>
>On Fri, Nov 03, 2017 at 11:00:05AM -0700, Megha Dey wrote:
>
>SNIP
>
>> +
>> +static void intel_bm_event_update(struct perf_event *event) {
>> +	union bm_detect_status cur_stat;
>> +
>> +	rdmsrl(BR_DETECT_STATUS_MSR, cur_stat.raw);
>> +	local64_set(&event->hw.prev_count, (uint64_t)cur_stat.raw); }
>> +
>> +static void intel_bm_event_stop(struct perf_event *event, int mode) {
>> +	wrmsrl(BR_DETECT_COUNTER_CONFIG_BASE + event->id,
>> +	       (event->hw.bm_counter_conf & ~1));
>> +
>> +	intel_bm_event_update(event);
>> +}
>> +
>> +static void intel_bm_event_del(struct perf_event *event, int flags) {
>> +	intel_bm_event_stop(event, flags);
>> +}
>> +
>> +static void intel_bm_event_read(struct perf_event *event) { }
>
>should you call intel_bm_event_update in here? so the read syscall gets
>updated data in case the case the event is active

We do not update the event->count in the intel_bm_event_update function. We are basically saving the status register contents when a task is being scheduled out. So it has nothing to do with the read syscall. Having said that, I will probably put what stop() does in del() and get rid of the stop() function.
>
>jirka

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