From f9494422b9c4abe8f7cfea0ecee729bbc618de02 Mon Sep 17 00:00:00 2001

From: David Hildenbrand <david@redhat.com>

Date: Fri, 21 Dec 2018 15:39:56 +0100

Subject: [PATCH 12/14] s390x/tod: Properly stop the KVM TOD while the guest is

 not running

RH-Author: David Hildenbrand <david@redhat.com>

Message-id: <20181221153957.28183-12-david@redhat.com>

Patchwork-id: 83765

O-Subject: [RHEL-7.6.z qemu-kvm-ma PATCH 11/12] s390x/tod: Properly stop the KVM TOD while the guest is not running

Bugzilla: 1672920

RH-Acked-by: Cornelia Huck <cohuck@redhat.com>

RH-Acked-by: Thomas Huth <thuth@redhat.com>

RH-Acked-by: Laurent Vivier <lvivier@redhat.com>

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>

Message-Id: <20181130094957.4121-1-david@redhat.com>

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>

Reviewed-by: Thomas Huth <thuth@redhat.com>

Signed-off-by: Cornelia Huck <cohuck@redhat.com>

(cherry picked from commit 9bc9d3d1ae3bcd1caaad1946494726b52f58b291)

Signed-off-by: David Hildenbrand <david@redhat.com>

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>

---

 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 df564ab..2456bf7 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;

+}

+

 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,

     .class_init = kvm_s390_tod_class_init,

     .class_size = sizeof(S390TODClass),

 };

diff --git a/include/hw/s390x/tod.h b/include/hw/s390x/tod.h

index 413c0d7..cbd7552 100644

--- a/include/hw/s390x/tod.h

+++ b/include/hw/s390x/tod.h

@@ -31,13 +31,19 @@ typedef struct S390TODState {

     /* private */

     DeviceState parent_obj;

-    /* unused by KVM implementation */

+    /*

+     * Used by TCG to remember the time base. Used by KVM to backup the TOD

+     * while the TOD is stopped.

+     */

     S390TOD base;

+    /* Used by KVM to remember if the TOD is stopped and base is valid. */

+    bool stopped;

 } S390TODState;

 typedef struct S390TODClass {

     /* private */

     DeviceClass parent_class;

+    void (*parent_realize)(DeviceState *dev, Error **errp);

     /* public */

     void (*get)(const S390TODState *td, S390TOD *tod, Error **errp);

-- 

1.8.3.1