KVM: x86: apply kvmclock offset to guest wall clock time

(cherry picked from commit 4b6486659a7defef82ea51b276024b3aa357fefc)

When a guest migrates to a new host, the system time difference from the
previous host is used in the updates to the kvmclock system time visible
to the guest, resulting in a continuation of correct kvmclock based guest
timekeeping.

The wall clock component of the kvmclock provided time is currently not
updated with this same time offset. Since the Linux guest caches the
wall clock based time, this discrepency is not noticed until the guest is
rebooted. After reboot the guest's time calculations are off.

This patch adjusts the wall clock by the kvmclock_offset, resulting in
correct guest time after a reboot.

Cc: Zachary Amsden <zamsden@gmail.com>
Signed-off-by: Bruce Rogers <brogers@suse.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index be6d54929fa7d661c31f65d076c0daad8086d785..14c290de7985b55f4a73c310e6636965ead9b4fd 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -907,6 +907,10 @@ static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock)
         */
        getboottime(&boot);
 
+       if (kvm->arch.kvmclock_offset) {
+               struct timespec ts = ns_to_timespec(kvm->arch.kvmclock_offset);
+               boot = timespec_sub(boot, ts);
+       }
        wc.sec = boot.tv_sec;
        wc.nsec = boot.tv_nsec;
        wc.version = version;