On Wed, Nov 14, 2007 at 10:44:20AM -0500, William Cohen wrote:
> Andi Kleen wrote:
>
> >>One approach does not prevent the other. Assuming you allow cr4.pce, then
> >>nothing prevents
> >>a self-monitoring thread from reading the counters directly. You'll just
> >>get the
> >>lower 32-bit of it. So if you read frequently enough, you should not have
> >>a problem.
> >
> >Hmm? RDPMC is 64bit.
>
> There are a number of processors that have 32-bit counters such as the IBM
> power processors. On many x86 processors the upper bits of the counter are
> sign extended from the lower 32 bits. Thus, one can only assume the lower
> 32-bit are available. Roll over of values is quite possible (<2 seconds of
> cycle count), so additional work needs to be done to obtain a valid value.
>
Exactly, on Intel's only the bottom 32-bit actually are useable, the rest is
sign-extension. That's why it is okay for measuring small sections of code,
but that's it. On AMD, I think it is better. On Itanium you get the 47-bit worth.
Don't know about Power or Cell.
--
-Stephane
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