Re: [Qemu-devel] [PATCH v1] s390x/tod: properly stop the KVM TOD while the guest is not running

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 27 Nov 2018 12:41:02 +0100
David Hildenbrand <david@redhat.com> wrote:

> Just like on other architectures, we should stop the clock while the guest
> is not running. This is already properly done for TCG. Right now, doing an
> offline migration (stop, migrate, cont) can easily trigger stalls in the
> guest.
> 
> Even doing a
>     (hmp) stop
>     ... wait 2 minutes ...
>     (hmp) cont
> will already trigger stalls.
> 
> So whenever the guest stops, backup the KVM TOD. When continuning to run
> the guest, restore the KVM TOD.
> 
> One special case is starting a simple VM: Reading the TOD from KVM to
> stop it right away until the guest is actually started means that the
> time of any simple VM will already differ to the host time. We can
> simply leave the TOD running and the guest won't be able to recognize
> it.
> 
> For migration, we actually want to keep the TOD stopped until really
> starting the guest. To be able to catch most errors, we should however
> try to set the TOD in addition to simply storing it. So we can still
> catch basic migration problems.
> 
> If anything goes wrong while backing up/restoring the TOD, we have to
> ignore it (but print a warning). This is then basically a fallback to
> old behavior (TOD remains running).
> 
> I tested this very basically with an initrd:
>     1. Start a simple VM. Observed that the TOD is kept running. Old
>        behavior.
>     2. Ordinary live migration. Observed that the TOD is temporarily
>        stopped on the destination when setting the new value and
>        correctly started when finally starting the guest.
>     3. Offline live migration. (stop, migrate, cont). Observed that the
>        TOD will be stopped on the source with the "stop" command. On the
>        destination, the TOD is temporarily stopped when setting the new
>        value and correctly started when finally starting the guest via
>        "cont".
>     4. Simple stop/cont correctly stops/starts the TOD. (multiple stops
>        or conts in a row have no effect, so works as expected)
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  hw/s390x/tod-kvm.c     | 88 +++++++++++++++++++++++++++++++++++++++++-
>  include/hw/s390x/tod.h |  7 +++-
>  2 files changed, 92 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/s390x/tod-kvm.c b/hw/s390x/tod-kvm.c
> index df564ab89c..d4b12e1145 100644
> --- a/hw/s390x/tod-kvm.c
> +++ b/hw/s390x/tod-kvm.c

> @@ -38,6 +49,58 @@ static void kvm_s390_tod_set(S390TODState *td, const S390TOD *tod, Error **errp)
>      if (r) {
>          error_setg(errp, "Unable to set KVM guest TOD clock: %s",
>                     strerror(-r));
> +        return;

I don't think you need this new 'return'.

> +    }
> +}

(...)

> +static void kvm_s390_tod_init(Object *obj)
> +{
> +    S390TODState *td = S390_TOD(obj);
> +
> +    /*
> +     * The TOD is initially running (value stored in KVM). Avoid needless
> +     * loading/storing of the TOD when starting a simple VM, so let it
> +     * run although the (never started) VM is stopped. For migration, we
> +     * will properly set the TOD later.
> +     */
> +    td->stopped = false;
> +
> +    /*
> +     * We need to know when the VM gets started/stopped to start/stop the TOD.
> +     * As we can never have more than one TOD instance (and that will never be
> +     * removed), registering here and never unregistering is good enough.
> +     */
> +    qemu_add_vm_change_state_handler(kvm_s390_tod_vm_state_change, td);
> +}
> +
>  static TypeInfo kvm_s390_tod_info = {
>      .name = TYPE_KVM_S390_TOD,
>      .parent = TYPE_S390_TOD,
>      .instance_size = sizeof(S390TODState),
> +    .instance_init = kvm_s390_tod_init,

See Thomas' comment wrt realize vs. instance_init.

>      .class_init = kvm_s390_tod_class_init,
>      .class_size = sizeof(S390TODClass),
>  };

This one is a bit late for 3.1, but should it be cc:stable?