Hi QEMU team, Any update for this kvmclock issue?
Thanks in advance. ---------------------------------------------------------------------- BW Pengju Zhou > -----Original Message----- > From: Peng Ju Zhou <[email protected]> > Sent: Thursday, November 20, 2025 4:44 PM > To: [email protected]; [email protected]; [email protected]; > [email protected]; [email protected] > Cc: [email protected]; Chang, HaiJun <[email protected]>; Ma, > Qing (Mark) <[email protected]>; Zhou, Peng Ju <[email protected]> > Subject: [PATCH] hw/i386/kvm: Prevent guest monotonic clock jump after live > migration > > Problem > After live migration, the guest monotonic clock may jump forward on the > target. > > Cause > kvmclock (the guest's time base) is derived from host wall time and keeps > advancing > while the VM is paused. During STOP_COPY, QEMU reads kvmclock twice: > 1) immediately after the VM is paused, and > 2) when final CPU state is collected. > Only the second (later) value is migrated. The gap between the two reads is > roughly > the downtime, so the target restores from a later time and the guest > monotonic clock > jumps ahead. > > Fix > Migrate the kvmclock value captured at pause time (the first read) so the > target > restores from the actual pause point. > > Signed-off-by: Peng Ju Zhou <[email protected]> > --- > hw/i386/kvm/clock.c | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c index > 40aa9a32c3..cd6f7e1315 > 100644 > --- a/hw/i386/kvm/clock.c > +++ b/hw/i386/kvm/clock.c > @@ -43,6 +43,7 @@ struct KVMClockState { > > /* whether the 'clock' value was obtained in the 'paused' state */ > bool runstate_paused; > + RunState state; > > /* whether machine type supports reliable KVM_GET_CLOCK */ > bool mach_use_reliable_get_clock; > @@ -108,7 +109,10 @@ static void kvm_update_clock(KVMClockState *s) > fprintf(stderr, "KVM_GET_CLOCK failed: %s\n", strerror(-ret)); > abort(); > } > - s->clock = data.clock; > + > + if (s->state != RUN_STATE_FINISH_MIGRATE) { > + s->clock = data.clock; > + } > > /* If kvm_has_adjust_clock_stable() is false, KVM_GET_CLOCK returns > * essentially CLOCK_MONOTONIC plus a guest-specific adjustment. This > @@ - > 217,6 +221,8 @@ static void kvmclock_vm_state_change(void *opaque, bool > running, > */ > s->clock_valid = true; > } > + > + s->state = state; > } > > static void kvmclock_realize(DeviceState *dev, Error **errp) > -- > 2.33.1
