Messages in this thread | | | Date | Tue, 01 Feb 2011 19:09:23 +0200 | From | Avi Kivity <> | Subject | Re: [PATCH v2 2/6] KVM-HV: KVM Steal time implementation |
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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.
-- error compiling committee.c: too many arguments to function
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