Re: [QEMU PATCH] kvmclock: advance clock by time window between vm_stop and pre_save

["Radim Krčmář" <rkrcmar@redhat.com>]

2016-11-04 07:43-0200, Marcelo Tosatti:
> This patch, relative to pre-copy migration codepath,
> measures the time between vm_stop() and pre_save(), 
> which includes copying the remaining RAM to destination,
> and advances the clock by that amount.
> 
> In a VM with 5 seconds downtime, this reduces the guest 
> clock difference on destination from 5s to 0.2s.
> 
> Please do not apply this yet as some codepaths still need
> checking, submitting early for comments.

The time computation looks ok.

We could make it slightly more precise by returning the CLOCK_MONOTONIC
at which KVM_GET_CLOCK is read with the IOCTL, but we don't account the
migration time anyway, so that precision would be wasted.

> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c
> @@ -100,6 +106,11 @@ static void kvmclock_vm_state_change(void *opaque, int running,
>              s->clock = time_at_migration;
>          }
>  
> +        if (s->advance_clock && s->clock + s->advance_clock > s->clock) {
> +            s->clock += s->advance_clock;
> +            s->advance_clock = 0;
> +        }

Can't the advance_clock added to the migrated KVMClockState instead of
passing it as another parameter?

(It is sad that we can't just query KVMClockState in kvmclock_pre_save
 because of the Linux bug.)

> @@ -135,6 +146,18 @@ static void kvmclock_vm_state_change(void *opaque, int running,
>              abort();
>          }
>          s->clock = data.clock;
> +        /*
> +         * Transition from VM-running to VM-stopped via migration?
> +         * Record when the VM was stopped.
> +         */
> +
> +        if (state == RUN_STATE_FINISH_MIGRATE &&
> +            !migration_in_postcopy(migrate_get_current())) {
> +            clock_gettime(CLOCK_MONOTONIC, &s->t_aftervmstop);

How big (more like small) was the clock delta between here and
kvmclock_pre_save with postcopy?

Thanks.