Re: [rfc 1/3] perf, x86: P4 PMU - describe config format

Stephane Eranian
2010-11-26T13:10:46Z

On Fri, Nov 26, 2010 at 2:04 PM, Cyrill Gorcunov <gorcunov@openvz.org> wrote:
> On 11/26/10, Stephane Eranian <eranian@google.com> wrote:
>> On Fri, Nov 26, 2010 at 12:58 PM, Cyrill Gorcunov <gorcunov@openvz.org>
>> wrote:
>>> On 11/26/10, Stephane Eranian <eranian@google.com> wrote:
>>>> On Fri, Nov 26, 2010 at 12:32 PM, Cyrill Gorcunov <gorcunov@openvz.org>
>>>> wrote:
>>>>> On Fri, Nov 26, 2010 at 2:14 PM, Cyrill Gorcunov <gorcunov@openvz.org>
>>>>> wrote:
>>>>>> Stephane, this is a misprint, I'll update this comments on format
>>>>>> (giod catch btw!). in real low 32 bits are considered as cccr in ht
>>>>>> mode. wait a bit, i'll post update.
>>>>>>
>>>>>> On 11/26/10, Stephane Eranian <eranian@google.com> wrote:
>>>>> ...
>>>>>>>> + *    Low 32 bits
>>>>>>>> + *    -----------
>>>>>>>> + *      0-6: P4_PEBS_METRIC enum
>>>>>>>> + *     7-11:                    reserved
>>>>>>>> + *       12: Active thread
>>>>>>>
>>>>>>> I don't understand bit 12. In the actual register, it
>>>>>>> corresponds to the enable bit. Seems you're overriding
>>>>>>> its usage. Do I interpret this as saying: 0 = enable when
>>>>>>> running on thread0, 1=monitoring when running on thread1?
>>>>>>> And if I don't care?
>>>>> ...
>>>>> I believe it simply escaped quilt refresh somehow. Here is the
>>>>> 'refreshed'
>>>>> copy (note the low bits 12-19 updated here).
>>>>> ---
>>>>> perf, x86: P4 PMU - describe config format v2
>>>>>
>>>>> Add description of .config in a sake of RAW events.
>>>>> At least this should bring some light to those who
>>>>> will be reading this code.
>>>>>
>>>>> Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
>>>>> CC: Lin Ming <ming.m.lin@intel.com>
>>>>> CC: Stephane Eranian <eranian@google.com>
>>>>> CC: Peter Zijlstra <peterz@infradead.org>
>>>>> ---
>>>>>  arch/x86/include/asm/perf_event_p4.h |   63
>>>>> ++++++++++++++++++++++++++++++-----
>>>>>  1 file changed, 55 insertions(+), 8 deletions(-)
>>>>>
>>>>> Index: linux-2.6.tip/arch/x86/include/asm/perf_event_p4.h
>>>>> ===================================================================
>>>>> --- linux-2.6.tip.orig/arch/x86/include/asm/perf_event_p4.h
>>>>> +++ linux-2.6.tip/arch/x86/include/asm/perf_event_p4.h
>>>>> @@ -744,14 +744,6 @@ enum P4_ESCR_EMASKS {
>>>>>  };
>>>>>
>>>>>  /*
>>>>> - * P4 PEBS specifics (Replay Event only)
>>>>> - *
>>>>> - * Format (bits):
>>>>> - *   0-6: metric from P4_PEBS_METRIC enum
>>>>> - *    7 : reserved
>>>>> - *    8 : reserved
>>>>> - * 9-11 : reserved
>>>>> - *
>>>>>  * Note we have UOP and PEBS bits reserved for now
>>>>>  * just in case if we will need them once
>>>>>  */
>>>>> @@ -788,5 +780,60 @@ enum P4_PEBS_METRIC {
>>>>>        P4_PEBS_METRIC__max
>>>>>  };
>>>>>
>>>>> +/*
>>>>> + * Notes on internal configuration of ESCR+CCCR tuples
>>>>> + *
>>>>> + * Since P4 has quite the different architecture of
>>>>> + * performance registers in compare with "architectural"
>>>>> + * once and we have on 64 bits to keep configuration
>>>>> + * of performance event, the following trick is used.
>>>>> + *
>>>>> + * 1) Since both ESCR and CCCR registers have only low
>>>>> + *    32 bits valuable, we pack them into a single 64 bit
>>>>> + *    configuration. Low 32 bits of such config correspond
>>>>> + *    to low 32 bits of CCCR register and high 32 bits
>>>>> + *    correspond to low 32 bits of ESCR register.
>>>>> + *
>>>>> + * 2) The meaning of every bit of such config field can
>>>>> + *    be found in Intel SDM but it should be noted that
>>>>> + *    we "borrow" some reserved bits for own usage and
>>>>> + *    clean them or set to a proper value when we do
>>>>> + *    a real write to hardware registers.
>>>>> + *
>>>>> + * 3) The format of bits of config is the following
>>>>> + *    and should be either 0 or set to some predefined
>>>>> + *    values:
>>>>> + *
>>>>> + *    Low 32 bits
>>>>> + *    -----------
>>>>> + *      0-6: P4_PEBS_METRIC enum
>>>>> + *     7-11:                    reserved
>>>>> + *       12:                    reserved (Enable)
>>>>> + *    13-15:                    reserved (ESCR select)
>>>>> + *    16-17: Active Thread
>>>>
>>>> HW has the active thread bits reserved to 0x3.
>>>> what about you? If not, then explain what they
>>>> mean.
>>>>
>>> hm, not sure i follow, hw allows you to pass any of 4 values for that
>>> field, so i simply pass it to kernel and then propagate to real cccr
>>> register. if machine is not ht capable it might be a problem, but i
>>> left it to a caller to set proper thread value here. i believe that
>>> you read cccr spec for non-ht machine while ht machine has a bit more
>>> flags to set.
>>>
>> You're right, I missed Figure-30.29. So you honor the field. The counter
>> won't count anything if the task is not running on the corresponding
>> HT thread, then.
>
> no ;) p4 treat this field in different meaning in compare to others
> perf archs. the condition is not 'where' but 'when'. ie you can
> specify 'when' to run counter: if none therad is active, single any
> thread is active, both threads are active and finally any thread is
> active (which is exactly 0b11 in single thread machine). i know it's
> weird ;)
>
That is weird indeed.

I can see this being somewhat useful in per-cpu mode. But this is
weird in per-thread mode. That means you will count in your thread
(OS and HW) only when the other HW thread runs. I am wondering
about the security implications of this. I suspect you'd have to
disallow this mode in per-thread mode.

> thanks for review, Stephane ;)
>
>>
>> The only custom fields are then: 0-6, 25-30. I think that's simple enough.
>>
>> Thanks.
>>
>