Re: [PATCH 1/2] kvm: kvmclock: apply kvmclock offset to guest wall clock time

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Jul 20, 2012 at 10:44:24AM -0600, Bruce Rogers wrote:
> 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: Glauber Costa <glommer@redhat.com>
> Cc: Zachary Amsden <zamsden@gmail.com>
> Signed-off-by: Bruce Rogers <brogers@suse.com>
> ---
>  arch/x86/kvm/x86.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index be6d549..14c290d 100644
> --- 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);
> +	}

kvmclock_offset is signed (both directions). Must check the sign and use
_sub and _add_safe accordingly.