Re: perf_counter Atom patch

[stephane eranian]

On Tue, Jun 23, 2009 at 11:10 AM, Peter Zijlstra<a.p.zijlstra@xxxxxxxxx> wrote:
> On Tue, 2009-06-23 at 16:34 +0800, Yong Wang wrote:
>> > you could simply consider having 0 fixed counters and everything else would work
>> > as expected. But there is a catch, unfortunately, in that there is erratum AE49
>> > which says that there is only one enable bit to control the two generic counters
>> > on Core Duo/Solo.
>
> Ah, that's similar to P6 like machines. The P6 docs say that to disable
> a counter you should simply write all zeros (except the EN bit for ctr0)
> to the control register (IIRC).
>
> I suppose we could do something similar on these errata cores, make
> x86_pmu_disable_counter() write ARCH_PERFMON_EVENTSEL0_ENABLE instead.
>
> Would that work?
>
I suspect that to make this work correctly on P6 and Core Duo,  you
will have to enforce
only one event/group to maintain the independence you expose at the
user level. An
Alternative would be to ensure that:
   - group leader in always in counter0
   - sibling events are created with disabled=0
   - ioctl(ENABLE/DISABLE) on siblings always fail

Of course, this does not work, if the group leader event requires
counter1. But I have to check
if such restriction exists on Core Duo.
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