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    SubjectRe: [PATCH RFC: kvm tsc virtualization 15/20] Fix longstanding races
    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).



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