Subject: Re: [PATCH v2 2/6] KVM-HV: KVM Steal time implementation
From: Glauber Costa <glommer@redhat.com>
To: Avi Kivity <avi@redhat.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, aliguori@us.ibm.com,
        Rik van Riel <riel@redhat.com>,
        Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>,
        Peter Zijlstra <peterz@infradead.org>
In-Reply-To: <4D483E43.5060904@redhat.com>
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	 <4D483E43.5060904@redhat.com>
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Organization: Red Hat
Date: Tue, 01 Feb 2011 17:58:30 -0200
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On Tue, 2011-02-01 at 19:09 +0200, Avi Kivity wrote:
> On 02/01/2011 05:48 PM, Glauber Costa wrote:
> > >  >  @@ -2106,6 +2120,25 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> > >  >    			kvm_migrate_timers(vcpu);
> > >  >    		vcpu->cpu = cpu;
> > >  >    	}
> > >  >  +
> > >  >  +	if (vcpu->arch.this_time_out) {
> > >  >  +		u64 to = (get_kernel_ns() - vcpu->arch.this_time_out);
> > >  >  +		/*
> > >  >  +		 * using nanoseconds introduces noise, which accumulates easily
> > >  >  +		 * leading to big steal time values. We want, however, to keep the
> > >  >  +		 * interface nanosecond-based for future-proofness.
> > >  >  +		 */
> > >  >  +		to /= NSEC_PER_USEC;
> > >  >  +		to *= NSEC_PER_USEC;
> > >
> > >  Seems there is a real problem and that this is just papering it over.
> > >  I'd like to understand the root cause.
> > Okay, my self-explanation seemed reasonable to me, but since both you
> > and Peter dislike it, I think it is important enough to get a more
> > thorough investigation before a second round.
> 
> Yes please.
> 
> > But in this case,
> > I keep that using nanoseconds may then not be the best approach here. We
> > also have to keep in mind that the host and guest clocks may be running
> > at different resolutions.
> 
> We need to choose a resolution for the clock (or negotiate one), an 
> nanoseconds seems as good as any from a range and precision 
> considerations, and is convenient for the host and Linux guests.  So why 
> not pick it?
> 
> > >  >  +		vcpu->arch.sversion += 2;
> > >
> > >  Doesn't survive live migration.  You need to use the version from the
> > >  guest area.
> > Why not? Who said versions need to always increase? If current version
> > is 102324, and we live migrate and it becomes 0, what is the problem?
> 
> Guest reads version (result: 2)
> Guest starts reading data
> Live migration; vcpu->arch.sversion is zeroed
> Steal time update; vcpu->arch.sversion += 2; write to guest
> Guest continues reading data
> Guest reads version (result: 2)
> 
> So the guest is unaware that an update has occurred while it was reading 
> the data.
Ok, fair.

By the way, kvmclock have the same problem, then
> 


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