linux-kernel - Re: [V2 PATCH 0/6] x86, NMI: give NMI handler a face-lift

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Message-ID: <1290112234.2109.1534.camel@laptop>
Date:	Thu, 18 Nov 2010 21:30:33 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Don Zickus <dzickus@...hat.com>
Cc:	Jason Wessel <jason.wessel@...driver.com>,
	Ingo Molnar <mingo@...e.hu>,
	Robert Richter <robert.richter@....com>, ying.huang@...el.com,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [V2 PATCH 0/6] x86, NMI: give NMI handler a face-lift

On Thu, 2010-11-18 at 15:08 -0500, Don Zickus wrote:
> On Thu, Nov 18, 2010 at 01:51:44PM -0600, Jason Wessel wrote:
> > > So the problem is when the nmi watchdog is enabled, the perf event is
> > > 'active' and thus tries to read the counter value.  Because it is always
> > > zero, perf just assumes the counter overflowed and the NMI is his.
> > >
> > > Not sure how to fix it yet, other than include the logic that detects we
> > > are on a guest and disable perf??
> > >
> > >   
> > 
> > I highly doubt we want to disable perf.   I would rather use the source
> > and fix the nmi emulation in KVM/Qemu after we hear back the results
> 
> Well I think Peter does not have a positive opinion about emulating perf
> inside a guest.  

Well, I'll let someone else write it.. I tihnk its pretty pointless to
have, the whole virt layer totally destroys many (if not all) useful
metrics.

But I don't have a problem with full msr emulation, what I do not like
is a direct msr passthough bypassing perf.

> Nor are the KVM folks having much success in doing so.

Just busy doing other stuff I guess.. Jes was going to prod at it at
some point.

> Just to clarify, perf counter emulation is _not_ implemented in kvm.
> Therefore disabling perf in the guest makes sense until someone gets
> around to actually writing the emulation code for perf in a guest. :-)

Right, which is what I proposed, on init do a checking_wrmsrl() on a
known PMU reg, KVM/qemu should fault on that.. (I'd prefer it if they'd
also fault on reading it too).


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