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

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Date:	Thu, 20 Jun 2013 10:35:36 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Igor Mammedov <imammedo@...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

Il 20/06/2013 10:30, Igor Mammedov ha scritto:
> 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.

Fine!  Please mention it (with --verbose flag) in the commit message.

Paolo

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