On Mon, May 20, 2019 at 05:43:40PM -0300, Maxiwell S. Garcia wrote: > This handler was added in the commit: > 42043e4f1241: spapr: clock should count only if vm is running > > In a scenario without migration, this pre_save handler is not > triggered, so the 'stop/cont' commands save and restore the clock > in the function 'cpu_ppc_clock_vm_state_change.' The SW clock > in the guest doesn't know about this pause. > > If the command 'migrate' is called between 'stop' and 'cont', > the pre_save handler re-read the clock, and the SW clock in the > guest will know about the pause between 'stop' and 'migrate.' > If the guest is running a workload like HTC, a side-effect of > this is a lot of process stall messages (with call traces) in > the kernel guest. > > Signed-off-by: Maxiwell S. Garcia <maxiw...@linux.ibm.com>
What affect will this have on the clock for the case of migrations
without a stop/cont around? The complicated thing here is that for
*explicit* stops/continues we want to freeze the clock, however for
the implicit stop/continue during migration downtime, we want to keep
the clock running (logically), so that the guest time of day doesn't
get out of sync on migration.
> ---
> hw/ppc/ppc.c | 24 ------------------------
> 1 file changed, 24 deletions(-)
>
> diff --git a/hw/ppc/ppc.c b/hw/ppc/ppc.c
> index ad20584f26..3fb50cbeee 100644
> --- a/hw/ppc/ppc.c
> +++ b/hw/ppc/ppc.c
> @@ -1056,35 +1056,11 @@ void cpu_ppc_clock_vm_state_change(void *opaque, int
> running,
> }
> }
>
> -/*
> - * When migrating, read the clock just before migration,
> - * so that the guest clock counts during the events
> - * between:
> - *
> - * * vm_stop()
> - * *
> - * * pre_save()
> - *
> - * This reduces clock difference on migration from 5s
> - * to 0.1s (when max_downtime == 5s), because sending the
> - * final pages of memory (which happens between vm_stop()
> - * and pre_save()) takes max_downtime.
Urgh.. this comment is confusing - 5s would be a ludicrously long
max_downtime by modern standards.
> - */
> -static int timebase_pre_save(void *opaque)
> -{
> - PPCTimebase *tb = opaque;
> -
> - timebase_save(tb);
> -
> - return 0;
> -}
> -
> const VMStateDescription vmstate_ppc_timebase = {
> .name = "timebase",
> .version_id = 1,
> .minimum_version_id = 1,
> .minimum_version_id_old = 1,
> - .pre_save = timebase_pre_save,
> .fields = (VMStateField []) {
> VMSTATE_UINT64(guest_timebase, PPCTimebase),
> VMSTATE_INT64(time_of_the_day_ns, PPCTimebase),
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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