Re: [Perfctr-devel] Re: Enabling RDPMC in user space by default

[David Gibson <david@gibson.dropbear.id.au>]

On Wed, Nov 30, 2005 at 01:34:33AM +0100, Andi Kleen wrote:
> On Wed, Nov 30, 2005 at 10:56:26AM +1100, David Gibson wrote:
> > On Wed, Nov 30, 2005 at 12:39:20AM +0100, Andi Kleen wrote:
> > > > Well, if that's all you want them to use RDPMC 0 for, why not just make
> > > > PMCs programmable from userspace?
> > > 
> > > First we need to have a cycle counter PMC anyways for the NMI watchdog.
> > > So it can as well be used for other purposes.
> > 
> > But the watchdog doesn't require it to be in the same place on all
> > machines.  Exporting it turns an internal requirement into an ABI
> > point.
> 
> Yes, but the only alternative I know of would be to let user space
> continue getting broken all the time with RDTSC.  RDPMC is a nice
> alternative. 

It just seems to me unwise to make an ABI commitment to something
that's not guaranteed by the architecture, perverse though it might
seem for the chip designers to take it away.  CPU designers have been
known to do some fairly perverse things from time to time..

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson