KVM: x86: Fix a possible backwards warp of kvmclock

Kernel time, which advances in discrete steps may progress much slower
than TSC.  As a result, when kvmclock is adjusted to a new base, the
apparent time to the guest, which runs at a much higher, nsec scaled
rate based on the current TSC, may have already been observed to have
a larger value (kernel_ns + scaled tsc) than the value to which we are
setting it (kernel_ns + 0).

We must instead compute the clock as potentially observed by the guest
for kernel_ns to make sure it does not go backwards.

Signed-off-by: Zachary Amsden <zamsden@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -339,6 +339,8 @@ struct kvm_vcpu_arch {
 	unsigned int time_offset;
 	struct page *time_page;
 	u64 last_host_tsc;
+	u64 last_guest_tsc;
+	u64 last_kernel_ns;
 
 	bool nmi_pending;
 	bool nmi_injected;