[Qemu-devel] [PATCH 1/2] make kvmclock value idempotent for stopped machine

[Glauber Costa <glommer@redhat.com>]

Although we never made such commitment clear (well, to the best
of my knowledge), some people expect that two savevm issued in sequence
in a stopped machine will yield the same results. This is not a crazy
requirement, since we don't expect a stopped machine to be updating its state,
for any device.

With kvmclock, this is not the case, since the .pre_save hook will issue an
ioctl to the host to acquire a timestamp, which is always changing.

This patch moves the value acquisition to vm_stop. This should mean our
get clock ioctl is issued more times, but this should be fine since vm_stop
is not a hot path.

When we do migrate, we'll transfer that value along.

Signed-off-by: Glauber Costa <glommer@redhat.com>
---
 cpus.c         |    4 ++++
 qemu-kvm-x86.c |    7 ++-----
 qemu-kvm.h     |    2 ++
 3 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/cpus.c b/cpus.c
index a55c330..879a03a 100644
--- a/cpus.c
+++ b/cpus.c
@@ -112,6 +112,10 @@ static void do_vm_stop(int reason)
         pause_all_vcpus();
         vm_state_notify(0, reason);
         monitor_protocol_event(QEVENT_STOP, NULL);
+        if (kvm_enabled()) {
+            kvmclock_update_clock();
+        }
+
     }
 }
 
diff --git a/qemu-kvm-x86.c b/qemu-kvm-x86.c
index 20b7d6d..d099d3d 100644
--- a/qemu-kvm-x86.c
+++ b/qemu-kvm-x86.c
@@ -500,11 +500,9 @@ static int kvm_enable_tpr_access_reporting(CPUState *env)
 #ifdef KVM_CAP_ADJUST_CLOCK
 static struct kvm_clock_data kvmclock_data;
 
-static void kvmclock_pre_save(void *opaque)
+void kvmclock_update_clock(void)
 {
-    struct kvm_clock_data *cl = opaque;
-
-    kvm_vm_ioctl(kvm_state, KVM_GET_CLOCK, cl);
+    kvm_vm_ioctl(kvm_state, KVM_GET_CLOCK, &kvmclock_data);
 }
 
 static int kvmclock_post_load(void *opaque, int version_id)
@@ -519,7 +517,6 @@ static const VMStateDescription vmstate_kvmclock= {
     .version_id = 1,
     .minimum_version_id = 1,
     .minimum_version_id_old = 1,
-    .pre_save = kvmclock_pre_save,
     .post_load = kvmclock_post_load,
     .fields      = (VMStateField []) {
         VMSTATE_U64(clock, struct kvm_clock_data),
diff --git a/qemu-kvm.h b/qemu-kvm.h
index 0f3fb50..b0b7ab3 100644
--- a/qemu-kvm.h
+++ b/qemu-kvm.h
@@ -752,6 +752,8 @@ int handle_tpr_access(void *opaque, CPUState *env, uint64_t rip,
 #define qemu_kvm_cpu_stop(env) do {} while(0)
 #endif
 
+void kvmclock_update_clock(void);
+
 #ifdef CONFIG_KVM
 
 typedef struct KVMSlot {
-- 
1.7.2.3