Re: [PATCH] hw/i386/kvm: Prevent guest monotonic clock jump after live migration

[Dongli Zhang <dongli.zhang@oracle.com>]

Hi Peng Ju,

On 11/20/25 12:44 AM, Peng Ju Zhou wrote:
> 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.

According to prior discussion, it is expected to account live migration downtime.

https://lore.kernel.org/qemu-devel/c1ceaa4e68b9264fc1c811c1ad0b60628d7fd9cd.camel@infradead.org/


That is, the jump forward is expected during live migration.


I used to send a QEMU patch to account live migration downtime.

[PATCH 1/1] target/i386/kvm: account blackout downtime for kvm-clock and guest TSC
https://lore.kernel.org/qemu-devel/20251009095831.46297-1-dongli.zhang@oracle.com/

Thank you very much!

Dongli Zhang

> 
> 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 <PengJu.Zhou@amd.com>
> ---
>  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)