Re: I.5 - Mmaped count

From: Paul Mackerras
Date: Mon Jun 22 2009 - 20:33:43 EST

stephane eranian writes:

> Unless you tell me that pc->index is marked invalid (0) when the
> event is not scheduled. I don't see how you can avoid reading
> the wrong value. I am assuming that is the event is not scheduled
> lock remains constant.

That's what happens; when pc->index == 0, the event is not scheduled
and the current count is in pc->offset.

> Assuming the event is active when you enter the loop and you
> read a value. How to get the timing information to scale the
> count?

At present you'd have to do a read(). It wouldn't be hard to add
fields to mmapped page to enable the user program to compute
up-to-date timing information from the TSC (x86) or timebase (powerpc)
value. We'll do that.

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