Before kernel commit 78db6a5037965429c04d708281f35a6e5562d31b, kvm_guest_time_update() would use vcpu->virtual_tsc_khz to calculate tsc_shift value in the vcpus pvclock structure written to guest memory. For those kernels, if vcpu->virtual_tsc_khz != tsc_khz (which can be the case when guest state is restored via migration, or if tsc-khz option is passed to QEMU), and TSC scaling is not enabled (which happens if the difference between the frequency requested via KVM_SET_TSC_KHZ and the host TSC KHZ is smaller than 250ppm), then there can be a difference between what KVM_GET_CLOCK would return and what the guest reads as kvmclock value. The effect is that the guest sees a jump in kvmclock value (either forwards or backwards) in such case. To fix incoming migration from pre-78db6a5037965 hosts, read kvmclock value from guest memory. Unless the KVM_CLOCK_CORRECT_TSC_SHIFT bit indicates that the value retrieved by KVM_GET_CLOCK on the source is safe to be used.
