On Thu, May 08, 2014 at 09:21:29AM +0200, Alexander Graf wrote: > > On 08.05.14 03:33, Marcelo Tosatti wrote: > >On Mon, May 05, 2014 at 03:51:22PM +0200, Alexander Graf wrote: > >>When we migrate we ask the kernel about its current belief on what the guest > >>time would be. However, I've seen cases where the kvmclock guest structure > >>indicates a time more recent than the kvm returned time. > >> > >>To make sure we never go backwards, calculate what the guest would have seen > >>as time at the point of migration and use that value instead of the kernel > >>returned one when it's more recent. > >> > >>While this doesn't fix the underlying issue that the kernel's view of time > >>is skewed, it allows us to safely migrate guests even from sources that are > >>known broken. > >> > >>Signed-off-by: Alexander Graf <agraf@xxxxxxx> > >OK Alexander better move this logic to the kernel, in KVM_GET_CLOCK. > > > >Otherwise every user of KVM_GET_CLOCK would have to apply the > >workaround. > > Well, the breakage occurs on the *source* of things. So if I have > 100 VMs running, I'm pretty sure one of them gets hit by this bug. > > If I put the workaround in the kernel, I have to take the chance to > break some of those 100 VMs to get things rolling. If I put it in > QEMU, I can live with a broken migration source. > > This gets even worse when you want to phase out unfixed host kernels. > > > Alex Alex, Unability to upgrade systems is not an excuse to fix the bug in the wrong place. I'll send a kernel patch. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
