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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Jul 23, 2012 at 09:44:54PM -0300, Marcelo Tosatti wrote:
> 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.

Your patch is correct, sorry (applied to master).

Patch 2 still makes no sense.