Messages in this thread | | | Date | Tue, 15 Dec 2009 11:26:59 -1000 | From | Zachary Amsden <> | Subject | Re: [PATCH RFC: kvm tsc virtualization 15/20] Fix longstanding races |
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On 12/15/2009 08:21 AM, Marcelo Tosatti wrote: > On Mon, Dec 14, 2009 at 06:08:42PM -1000, Zachary Amsden wrote: > > <snip> > > + atomic_set(&per_cpu(cpu_tsc_synchronized, freq->cpu), > 0); > + spin_lock(&kvm_lock); > + list_for_each_entry(kvm,&vm_list, vm_list) { > + kvm_for_each_vcpu(i, vcpu, kvm) { > + if (vcpu->cpu != freq->cpu) > + continue; > + if (vcpu->cpu != smp_processor_id()) > + send_ipi++; > + kvm_request_guest_time_update(vcpu); > > There is some overlap here between KVM_REQ_KVMCLOCK_UPDATE and > cpu_tsc_synchronized. Its the same information (frequency for a CPU has > changed) stored in two places. > > Later you do: > > spin_lock(&kvm_lock); > list_for_each_entry(kvm,&vm_list, vm_list) { > kvm_for_each_vcpu(i, vcpu, kvm) { > if (vcpu->cpu != freq->cpu) > continue; > if (vcpu->cpu != smp_processor_id()) > send_ipi++; > kvm_request_guest_time_update(vcpu); > } > } > spin_unlock(&kvm_lock); > <--- a remote CPU could have updated kvmclock information > with stale cpu_tsc_khz, clearing the > KVM_REQ_KVMCLOCK_UPDATE bit. > smp_call_function(evict) (which sets cpu_tsc_synchronized > to zero) > > Maybe worthwhile to unify it. Perhaps use the per cpu tsc generation in > addition to vcpu_load to update kvmclock info (on arch vcpu_load update > kvmclock store generation, update again on generation change). >
Yes, that is an excellent point. The generation counter, the tsc_synchronized variable and the per-vcpu clock counter all have some amount of redundancy of information.
Perhaps instead of overlapping, they should be layered?
A rule for kvmclock: can't update kvmclock info until cpu is synchronized?
Thanks,
Zach
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