Public bug reported:

1. when hot plug vcpu, KVM will call kvm_write_tsc set the new vcpu's
tsc_offset to host tsc.  The new vcpu's tsc_offsets is greatly different
from preallocated vcpu's tsc_offset.

2. For redhat 6.x, the vm will show warning and turn off TSC clock.
kernel: CPU 1 got hotplugged
kernel: kvm-clock: cpu 1, msr 0:282367c1, secondary cpu clock
kernel: TSC synchronization [CPU#0 -> CPU#1]:
kernel: Measured 150076598781 cycles TSC warp between CPUs, turning off TSC 
clock.
kernel: Marking TSC unstable due to check_tsc_sync_source failed

3. I think should set the new vcpu's tsc_offset to
kvm->arch.cur_tsc_offset.

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

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https://bugs.launchpad.net/bugs/1567167

Title:
  Ubuntu 15.10: tsc_offset is error for hot-plugged vcpu.

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