kernel-packages team mailing list archive

[changlimin <changlimin@xxxxxxx>]

I have tested 4.6-rc4, the bug exists.

** Changed in: linux (Ubuntu)
       Status: Incomplete => Confirmed

** Tags removed: kernel-da-key
** Tags added: kernel-bug-exists-upstream

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https://bugs.launchpad.net/bugs/1567159

Title:
  Ubuntu 15.10: After hot plug vcpu, kvm_arch->use_master_clock will
  never be true.

Status in linux package in Ubuntu:
  Confirmed

Bug description:
  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;

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