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

[Stephane Eranian]

On Fri, Nov 26, 2010 at 2:04 PM, Cyrill Gorcunov <gorcunov@xxxxxxxxxx> wrote:
> On 11/26/10, Stephane Eranian <eranian@xxxxxxxxxx> wrote:
>> On Fri, Nov 26, 2010 at 12:58 PM, Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
>> wrote:
>>> On 11/26/10, Stephane Eranian <eranian@xxxxxxxxxx> wrote:
>>>> On Fri, Nov 26, 2010 at 12:32 PM, Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
>>>> wrote:
>>>>> On Fri, Nov 26, 2010 at 2:14 PM, Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
>>>>> 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@xxxxxxxxxx> wrote:
>>>>> ...
>>>>>>>> + * Â ÂLow 32 bits
>>>>>>>> + * Â Â-----------
>>>>>>>> + * Â Â Â0-6: P4_PEBS_METRIC enum
>>>>>>>> + * Â Â 7-11: Â Â Â Â Â Â Â Â Â Âreserved
>>>>>>>> + * Â Â Â 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@xxxxxxxxxx>
>>>>> CC: Lin Ming <ming.m.lin@xxxxxxxxx>
>>>>> CC: Stephane Eranian <eranian@xxxxxxxxxx>
>>>>> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>>>>> ---
>>>>> Âarch/x86/include/asm/perf_event_p4.h | Â 63
>>>>> ++++++++++++++++++++++++++++++-----
>>>>> Â1 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 {
>>>>> Â};
>>>>>
>>>>> Â/*
>>>>> - * P4 PEBS specifics (Replay Event only)
>>>>> - *
>>>>> - * Format (bits):
>>>>> - * Â 0-6: metric from P4_PEBS_METRIC enum
>>>>> - * Â Â7 : reserved
>>>>> - * Â Â8 : reserved
>>>>> - * 9-11 : reserved
>>>>> - *
>>>>> Â* Note we have UOP and PEBS bits reserved for now
>>>>> Â* just in case if we will need them once
>>>>> Â*/
>>>>> @@ -788,5 +780,60 @@ enum P4_PEBS_METRIC {
>>>>> Â Â Â ÂP4_PEBS_METRIC__max
>>>>> Â};
>>>>>
>>>>> +/*
>>>>> + * 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
>>>>> + * Â Â32 bits valuable, we pack them into a single 64 bit
>>>>> + * Â Âconfiguration. Low 32 bits of such config correspond
>>>>> + * Â Âto low 32 bits of CCCR register and high 32 bits
>>>>> + * Â Âcorrespond to low 32 bits of ESCR register.
>>>>> + *
>>>>> + * 2) The meaning of every bit of such config field can
>>>>> + * Â Âbe found in Intel SDM but it should be noted that
>>>>> + * Â Âwe "borrow" some reserved bits for own usage and
>>>>> + * Â Âclean them or set to a proper value when we do
>>>>> + * Â Âa real write to hardware registers.
>>>>> + *
>>>>> + * 3) The format of bits of config is the following
>>>>> + * Â Âand should be either 0 or set to some predefined
>>>>> + * Â Âvalues:
>>>>> + *
>>>>> + * Â ÂLow 32 bits
>>>>> + * Â Â-----------
>>>>> + * Â Â Â0-6: P4_PEBS_METRIC enum
>>>>> + * Â Â 7-11: Â Â Â Â Â Â Â Â Â Âreserved
>>>>> + * Â Â Â 12: Â Â Â Â Â Â Â Â Â Âreserved (Enable)
>>>>> + * Â Â13-15: Â Â Â Â Â Â Â Â Â Âreserved (ESCR select)
>>>>> + * Â Â16-17: 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.
>>
>
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