Re: [Qemu-devel] [qemu-s390x] [PATCH v3] s390x/tod: Properly stop the KVM TOD while the guest is not running

[David Hildenbrand <david@redhat.com>]

On 30.11.18 13:39, Christian Borntraeger wrote:
> 
> 
> On 30.11.2018 10:49, David Hildenbrand 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 continuing 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)
>>
>> In the future, we might want to send the guest a special kind of time sync
>> interrupt under some conditions, so it can synchronize its tod to the
>> host tod. This is interesting for migration scenarios but also when we
>> get time sync interrupts ourselves. This however will most probably have
>> to be handled in KVM (e.g. when the tods differ too much) and is not
>> desired e.g. when debugging the guest. (single stepping should not
>> result in permanent time syncs). I consider something like that an add-on
>> on top of this basic "don't break the guest" handling.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>
>> v2 -> v3:
>> - use device_class_set_parent_realize() to implement a child realize
>>   function
>>
>>  hw/s390x/tod-kvm.c     | 102 ++++++++++++++++++++++++++++++++++++++++-
>>  include/hw/s390x/tod.h |   8 +++-
>>  2 files changed, 107 insertions(+), 3 deletions(-)
>>
>> diff --git a/hw/s390x/tod-kvm.c b/hw/s390x/tod-kvm.c
>> index df564ab89c..2456bf7b24 100644
>> --- a/hw/s390x/tod-kvm.c
>> +++ b/hw/s390x/tod-kvm.c
>> @@ -10,10 +10,11 @@
>>  
>>  #include "qemu/osdep.h"
>>  #include "qapi/error.h"
>> +#include "sysemu/sysemu.h"
>>  #include "hw/s390x/tod.h"
>>  #include "kvm_s390x.h"
>>  
>> -static void kvm_s390_tod_get(const S390TODState *td, S390TOD *tod, Error **errp)
>> +static void kvm_s390_get_tod_raw(S390TOD *tod, Error **errp)
>>  {
>>      int r;
>>  
>> @@ -27,7 +28,17 @@ static void kvm_s390_tod_get(const S390TODState *td, S390TOD *tod, Error **errp)
>>      }
>>  }
>>  
>> -static void kvm_s390_tod_set(S390TODState *td, const S390TOD *tod, Error **errp)
>> +static void kvm_s390_tod_get(const S390TODState *td, S390TOD *tod, Error **errp)
>> +{
>> +    if (td->stopped) {
>> +        *tod = td->base;
>> +        return;
>> +    }
>> +
>> +    kvm_s390_get_tod_raw(tod, errp);
>> +}
>> +
>> +static void kvm_s390_set_tod_raw(const S390TOD *tod, Error **errp)
>>  {
>>      int r;
>>  
>> @@ -41,18 +52,105 @@ static void kvm_s390_tod_set(S390TODState *td, const S390TOD *tod, Error **errp)
>>      }
>>  }
>>  
>> +static void kvm_s390_tod_set(S390TODState *td, const S390TOD *tod, Error **errp)
>> +{
>> +    Error *local_err = NULL;
>> +
>> +    /*
>> +     * Somebody (e.g. migration) set the TOD. We'll store it into KVM to
>> +     * properly detect errors now but take a look at the runstate to decide
>> +     * whether really to keep the tod running. E.g. during migration, this
>> +     * is the point where we want to stop the initially running TOD to fire
>> +     * it back up when actually starting the migrated guest.
>> +     */
>> +    kvm_s390_set_tod_raw(tod, &local_err);
>> +    if (local_err) {
>> +        error_propagate(errp, local_err);
>> +        return;
>> +    }
>> +
>> +    if (runstate_is_running()) {
>> +        td->stopped = false;
>> +    } else {
>> +        td->stopped = true;
>> +        td->base = *tod;
>> +    }
>> +}
>> +
>> +static void kvm_s390_tod_vm_state_change(void *opaque, int running,
>> +                                         RunState state)
>> +{
>> +    S390TODState *td = opaque;
>> +    Error *local_err = NULL;
>> +
>> +    if (running && td->stopped) {
>> +        /* Set the old TOD when running the VM - start the TOD clock. */
>> +        kvm_s390_set_tod_raw(&td->base, &local_err);
>> +        if (local_err) {
>> +            warn_report_err(local_err);
>> +        }
>> +        /* Treat errors like the TOD was running all the time. */
>> +        td->stopped = false;
>> +    } else if (!running && !td->stopped) {
>> +        /* Store the TOD when stopping the VM - stop the TOD clock. */
>> +        kvm_s390_get_tod_raw(&td->base, &local_err);
>> +        if (local_err) {
>> +            /* Keep the TOD running in case we could not back it up. */
>> +            warn_report_err(local_err);
>> +        } else {
>> +            td->stopped = true;
>> +        }
>> +    }
>> +}
>> +
>> +static void kvm_s390_tod_realize(DeviceState *dev, Error **errp)
>> +{
>> +    S390TODState *td = S390_TOD(dev);
>> +    S390TODClass *tdc = S390_TOD_GET_CLASS(td);
>> +    Error *local_err = NULL;
>> +
>> +    tdc->parent_realize(dev, &local_err);
>> +    if (local_err) {
>> +        error_propagate(errp, local_err);
>> +        return;
>> +    }
>> +
>> +    /*
>> +     * 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 void kvm_s390_tod_class_init(ObjectClass *oc, void *data)
>>  {
>>      S390TODClass *tdc = S390_TOD_CLASS(oc);
>>  
>> +    device_class_set_parent_realize(DEVICE_CLASS(oc), kvm_s390_tod_realize,
>> +                                    &tdc->parent_realize);
>>      tdc->get = kvm_s390_tod_get;
>>      tdc->set = kvm_s390_tod_set;
>>  }
>>  
>> +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;
> 
> 
> Do we have to migrate the td->stopped during migration?
> 

No, this is fully handled using the runstate.

-- 

Thanks,

David / dhildenb