linux-kernel - Re: [PATCH 4/4] kvm: Implement PEBS virtualization

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Message-ID: <20140602165231.GP25366@two.firstfloor.org>
Date:	Mon, 2 Jun 2014 18:52:31 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Gleb Natapov <gleb@...nel.org>
Cc:	Andi Kleen <ak@...ux.intel.com>, Andi Kleen <andi@...stfloor.org>,
	peterz@...radead.org, pbonzini@...hat.com, eranian@...gle.com,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	mtosatti@...hat.com
Subject: Re: [PATCH 4/4] kvm: Implement PEBS virtualization


BTW I found some more problems in the v1 version.

> > > With EPT it is less likely to happen (but still possible IIRC depending on memory
> > > pressure and how much memory shadow paging code is allowed to use), without EPT
> > > it will happen for sure.
> > 
> > Don't care about the non EPT case, this is white listed only for EPT supporting 
> > CPUs.
> User may still disable EPT during module load, so pebs should be dropped
> from a guest's cpuid in this case.

Ok.

> 
> > 
> > > There is nothing, as far as I can see, that says what will happen if the
> > > condition is not met. I always interpreted it as undefined behaviour so
> > > anything can happen including CPU dies completely.  You are saying above
> > > on one hand that CPU cannot handle any kinds of faults during write to
> > > DS area, but on the other hand a guest could only crash itself. Is this
> > > architecturally guarantied?
> > 
> > You essentially would get random page faults, and the PEBS event will
> > be cancelled. No hangs.
> Is this a guest who will get those random page faults or a host?

The guest (on the white listed CPU models)

-Andi
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