[PATCH 0/2] read kvmclock from guest memory if !correct_tsc_shift

[Marcelo Tosatti <mtosatti@redhat.com>]

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.