linux-kernel - Re: [PATCH 1/3] perf, core: Introduce attrs to count in either host or guest mode

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Date:	Tue, 10 May 2011 17:38:55 +0200
From:	"Roedel, Joerg" <Joerg.Roedel@....com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH 1/3] perf, core: Introduce attrs to count in either host
 or guest mode

On Tue, May 10, 2011 at 11:18:29AM -0400, Peter Zijlstra wrote:
> On Tue, 2011-05-10 at 16:59 +0200, Roedel, Joerg wrote:
> > On Tue, May 10, 2011 at 10:45:46AM -0400, Peter Zijlstra wrote:
> > > On Tue, 2011-05-10 at 16:35 +0200, Joerg Roedel wrote:
> > > > +       /* Can't exclude counting in guest and in host mode */
> > > > +       if (attr.exclude_host && attr.exclude_guest)
> > > > +               return -EINVAL;
> > > 
> > > Why not?
> > 
> > By definition the counter won't count at all. The hardware just ignores
> > the bits if they are both set. My rationale here was that it does not
> > makes sense to setup a counter and exclude guest and host mode.
> 
> I would expect it to 'work' but simply return 0. If that isn't what the
> AMD hardware does you need to fix that in the AMD driver.

By 'work' you mean that userspace can set it up but it doesn't count at
all in this situation? This would certainly be consistent behavior but I
can't imagine any use-case for it so that this code assumes that such a
situation is most likely a bug.
I can certainly change that if wanted, but I think its better to inform
userspace if we get weird values?

Regards,

	Joerg


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