• Glauber Costa's avatar
    KVM: allow userspace to adjust kvmclock offset · afbcf7ab
    Glauber Costa authored
    When we migrate a kvm guest that uses pvclock between two hosts, we may
    suffer a large skew. This is because there can be significant differences
    between the monotonic clock of the hosts involved. When a new host with
    a much larger monotonic time starts running the guest, the view of time
    will be significantly impacted.
    
    Situation is much worse when we do the opposite, and migrate to a host with
    a smaller monotonic clock.
    
    This proposed ioctl will allow userspace to inform us what is the monotonic
    clock value in the source host, so we can keep the time skew short, and
    more importantly, never goes backwards. Userspace may also need to trigger
    the current data, since from the first migration onwards, it won't be
    reflected by a simple call to clock_gettime() anymore.
    
    [marcelo: future-proof abi with a flags field]
    [jan: fix KVM_GET_CLOCK by clearing flags field instead of checking it]
    Signed-off-by: default avatarGlauber Costa <glommer@redhat.com>
    Signed-off-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
    Signed-off-by: default avatarAvi Kivity <avi@redhat.com>
    afbcf7ab
api.txt 20.7 KB