[QEMU PATCH] kvmclock: advance clock by time window between vm_stop and pre_save

[Marcelo Tosatti <mtosatti@redhat.com>]

This patch, relative to pre-copy migration codepath,
measures the time between vm_stop() and pre_save(), 
which includes copying the remaining RAM to destination,
and advances the clock by that amount.

In a VM with 5 seconds downtime, this reduces the guest 
clock difference on destination from 5s to 0.2s.

Please do not apply this yet as some codepaths still need
checking, submitting early for comments.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c
index 0f75dd3..1bd8fd6 100644
--- a/hw/i386/kvm/clock.c
+++ b/hw/i386/kvm/clock.c
@@ -22,9 +22,11 @@
 #include "kvm_i386.h"
 #include "hw/sysbus.h"
 #include "hw/kvm/clock.h"
+#include "migration/migration.h"
 
 #include <linux/kvm.h>
 #include <linux/kvm_para.h>
+#include <time.h>
 
 #define TYPE_KVM_CLOCK "kvmclock"
 #define KVM_CLOCK(obj) OBJECT_CHECK(KVMClockState, (obj), TYPE_KVM_CLOCK)
@@ -35,7 +37,11 @@ typedef struct KVMClockState {
     /*< public >*/
 
     uint64_t clock;
+    uint64_t ns;
     bool clock_valid;
+
+    uint64_t advance_clock;
+    struct timespec t_aftervmstop;
 } KVMClockState;
 
 struct pvclock_vcpu_time_info {
@@ -100,6 +106,11 @@ static void kvmclock_vm_state_change(void *opaque, int running,
             s->clock = time_at_migration;
         }
 
+        if (s->advance_clock && s->clock + s->advance_clock > s->clock) {
+            s->clock += s->advance_clock;
+            s->advance_clock = 0;
+        }
+
         data.clock = s->clock;
         ret = kvm_vm_ioctl(kvm_state, KVM_SET_CLOCK, &data);
         if (ret < 0) {
@@ -135,6 +146,18 @@ static void kvmclock_vm_state_change(void *opaque, int running,
             abort();
         }
         s->clock = data.clock;
+        /*
+         * Transition from VM-running to VM-stopped via migration?
+         * Record when the VM was stopped.
+         */
+
+        if (state == RUN_STATE_FINISH_MIGRATE &&
+            !migration_in_postcopy(migrate_get_current())) {
+            clock_gettime(CLOCK_MONOTONIC, &s->t_aftervmstop);
+        } else {
+            s->t_aftervmstop.tv_sec = 0;
+            s->t_aftervmstop.tv_nsec = 0;
+        }
 
         /*
          * If the VM is stopped, declare the clock state valid to
@@ -152,12 +175,66 @@ static void kvmclock_realize(DeviceState *dev, Error **errp)
     qemu_add_vm_change_state_handler(kvmclock_vm_state_change, s);
 }
 
+static uint64_t clock_delta(struct timespec *before, struct timespec *after)
+{
+    if (before->tv_sec > after->tv_sec ||
+        (before->tv_sec == after->tv_sec &&
+         before->tv_nsec > after->tv_nsec)) {
+        fprintf(stderr, "clock_delta failed: before=(%ld sec, %ld nsec),"
+                        "after=(%ld sec, %ld nsec)\n", before->tv_sec,
+                        before->tv_nsec, after->tv_sec, after->tv_nsec);
+        abort();
+    }
+
+    return (after->tv_sec - before->tv_sec) * 1000000000ULL +
+            after->tv_nsec - before->tv_nsec;
+}
+
+static void kvmclock_pre_save(void *opaque)
+{
+    KVMClockState *s = opaque;
+    struct timespec now;
+    uint64_t ns;
+
+    if (s->t_aftervmstop.tv_sec == 0) {
+        return;
+    }
+
+    clock_gettime(CLOCK_MONOTONIC, &now);
+
+    ns = clock_delta(&s->t_aftervmstop, &now);
+
+    /*
+     * Linux guests can overflow if time jumps
+     * forward in large increments.
+     * Cap maximum adjustment to 10 minutes.
+     */
+    ns = MIN(ns, 600000000000ULL);
+
+    if (s->clock + ns > s->clock) {
+        s->ns = ns;
+    }
+}
+
+static int kvmclock_post_load(void *opaque, int version_id)
+{
+    KVMClockState *s = opaque;
+
+    /* save the value from incoming migration */
+    s->advance_clock = s->ns;
+
+    return 0;
+}
+
 static const VMStateDescription kvmclock_vmsd = {
     .name = "kvmclock",
-    .version_id = 1,
+    .version_id = 2,
     .minimum_version_id = 1,
+    .pre_save = kvmclock_pre_save,
+    .post_load = kvmclock_post_load,
     .fields = (VMStateField[]) {
         VMSTATE_UINT64(clock, KVMClockState),
+        VMSTATE_UINT64_V(ns, KVMClockState, 2),
         VMSTATE_END_OF_LIST()
     }
 };