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

Cyrill Gorcunov
2010-11-26T13:04:12Z

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 ;)

thanks for review, Stephane ;)

>
> The only custom fields are then: 0-6, 25-30. I think that's simple enough.
>
> Thanks.
>