Re: [RFC][PATCH 08/11] perf, x86: Implement simple LBR support

From: Peter Zijlstra
Date: Thu Mar 04 2010 - 03:58:25 EST

On Wed, 2010-03-03 at 22:52 +0100, Stephane Eranian wrote:
> On Wed, Mar 3, 2010 at 5:39 PM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> > Implement support for Intel LBR stacks that support
> > FREEZE_LBRS_ON_PMI. We do not (yet?) support the LBR config register
> > because that is SMT wide and would also put undue restraints on the
> > PEBS users.
> >
> You're saying PEBS users have priorities over pure LBR users?
> Why is that?

I say no such thing, I only say it would make scheduling the PEBS things
more interesting.

> Without coding this, how would you expose LBR configuration to userland
> given you're using the PERF_SAMPLE_BRANCH_STACK approach?

Possibly using a second config word in the attr, but given how sucky the
hardware currently is (sharing the config between SMT) I'd be inclined
to pretend it doesn't exist for the moment.

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