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[Launchpad Bug Tracker <1567159@xxxxxxxxxxxxxxxxxx>]

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1. In function kvm_write_tsc, when tsc first matched, do
kvm->arch.nr_vcpus_matched_tsc++;

2. In function pvclock_update_vm_gtod_copy, if vcpus_matched (also with
other conditions meet), ka->use_master_clock will be true;

3. After hot plug vcpu, kvm_arch_vcpu_postcreate and Qemu will call
kvm_write_tsc for the new vcpu. Because the tsc will not match, so do
kvm->arch.nr_vcpus_matched_tsc = 0;

4. For other already existed vcpu will not call kvm_write_tsc, so will
not do kvm->arch.nr_vcpus_matched_tsc++;

5. So vcpus_matched in function pvclock_update_vm_gtod_copy will be
false forever, this cause ka->use_master_clock will never be true;

** Affects: linux (Ubuntu)
     Importance: Undecided
         Status: New

-- 
Ubuntu 15.10: After hot plug vcpu, kvm_arch->use_master_clock will never be true.
https://bugs.launchpad.net/bugs/1567159
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