linux-kernel - Re: [PATCHv1] kvm guest: fix uninitialized kvmclock read by KVM guest

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Message-ID: <20130620103004.4aa39c47@nial.usersys.redhat.com>
Date:	Thu, 20 Jun 2013 10:30:04 +0200
From:	Igor Mammedov <imammedo@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Batalov Eugene <eabatalov89@...il.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Eugene Batalov <ebatalov@...allels.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv1] kvm guest: fix uninitialized kvmclock read by KVM
 guest

On Wed, 19 Jun 2013 15:29:31 +0200
Paolo Bonzini <pbonzini@...hat.com> wrote:

> Il 19/06/2013 15:20, Batalov Eugene ha scritto:
> > 
> > I've missed this detail. It looks like Igor's patch doesn't bring
> > secondary cpus kvm_clocksource behavior back to one before the regression,
> > Before the regression per_cpu variables are used to allocate
> > kvm_pv_clock areas.
> > To to usage of percpu variables bootstrap cpu kvm_clock area contents
> > were copied to smp secondary cpus kvm_clock areas when they were started.
> > Bootstrap cpu kvm_clock area was not zeroed at this time.
> > So kvm_pv_clock for secondary cpus never returned "zero" clock before
> > the regression.
> > 
> > During the analysis of the bug I introduced idea to return zero before
> > kvm clocksource is initialized for secondary cpus
> > just like bootstrap cpu does on kernel boot. You can read that in BZ.
> 
> Yes, this is why I prefer to invert the two function calls.  But Igor's
> patch fixes the hang (trivially because version is even) and is more
> appropriate for -rc6.

I'll post this swap shortly, but zeroing out hv_clock at init time,
would be still needed to provide sane values there if ftrace enabled
at that time.

> 
> Paolo
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