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SubjectRe: II.1 - Fixed counters on Intel
On Mon, Jun 22, 2009 at 1:57 PM, Ingo Molnar<> wrote:
>> II/ X86 comments
>>  1/ Fixed counters on Intel
>> You cannot simply fall back to generic counters if you cannot find
>> a fixed counter. There are model-specific bugs, for instance
>> UNHALTED_REFERENCE_CYCLES (0x013c), does not measure the same
>> thing on Nehalem when it is used in fixed counter 2 or a generic
>> counter. The same is true on Core.
> This could be handled via a model specific quirk, if the erratum is
> serious enough.

Better demonstrated with an actual example on a 2.4GHz Quad: 10s noploop

$ pfmon -v --us-c
-u noploop 10
[FIXED_CTRL(pmc16)=0xaa0 pmi0=1 en0=0x0 pmi1=1 en1=0x2 pmi2=1 en2=0x2]
[PERFEVTSEL0(pmc0)=0x51013c event_sel=0x3c umask=0x1 os=0 usr=1 en=1
int=1 inv=0 edge=0 cnt_mask=0] CPU_CLK_UNHALTED
noploop for 10 seconds
2,650,345,853 CPU_CLK_UNHALTED:BUS

0x013c on fixed counter2 = unhalted_reference_cycles
0x013c on generic counter = cpu_clk_unhalted:bus

Difference is significant, isn't it?

And the question becomes: how do I express that I want the fixed counter version
of the event using the current PCL API, given both have the same encoding?

>> You cannot simply look at the event field code to determine
>> whether this is an event supported by a fixed counters. You must
>> look at the other fields such as edge, invert, cnt-mask. If those
>> are present then you have to fall back to using a generic counter
>> as fixed counters only support priv level filtering. As indicated
>> above, though, programming UNHALTED_REFERENCE_CYCLES on a generic
>> counter does not count the same thing, therefore you need to fail
>> if filters other than priv levels are present on this event.
> Agreed, we'll fix this.
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