HPET saves its state by calculating the current time and recovers timer
offset using this calculated value. But these calculations include
divisions and multiplications. Therefore the timer state cannot be recovered
precise enough.
This patch introduces saving of the original value of the offset to
preserve the determinism of the timer.

Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
Signed-off-by: Maria Klimushenkova <maria.klimushenkova@ispras.ru>
Reviewed-by: Juan Quintela <quintela@redhat.com>

--
v3: Added compat property for correct migration.
---
 hw/timer/hpet.c     |   30 ++++++++++++++++++++++++++++--
 include/hw/compat.h |    6 +++++-
 2 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/hw/timer/hpet.c b/hw/timer/hpet.c
index 577371b..d97436b 100644
--- a/hw/timer/hpet.c
+++ b/hw/timer/hpet.c
@@ -70,6 +70,7 @@ typedef struct HPETState {
 
     MemoryRegion iomem;
     uint64_t hpet_offset;
+    bool hpet_offset_saved;
     qemu_irq irqs[HPET_NUM_IRQ_ROUTES];
     uint32_t flags;
     uint8_t rtc_irq_level;
@@ -221,7 +222,9 @@ static int hpet_pre_save(void *opaque)
     HPETState *s = opaque;
 
     /* save current counter value */
-    s->hpet_counter = hpet_get_ticks(s);
+    if (hpet_enabled(s)) {
+        s->hpet_counter = hpet_get_ticks(s);
+    }
 
     return 0;
 }
@@ -252,7 +255,10 @@ static int hpet_post_load(void *opaque, int version_id)
     HPETState *s = opaque;
 
     /* Recalculate the offset between the main counter and guest time */
-    s->hpet_offset = ticks_to_ns(s->hpet_counter) - qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
+    if (!s->hpet_offset_saved) {
+        s->hpet_offset = ticks_to_ns(s->hpet_counter)
+                        - qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
+    }
 
     /* Push number of timers into capability returned via HPET_ID */
     s->capability &= ~HPET_ID_NUM_TIM_MASK;
@@ -267,6 +273,13 @@ static int hpet_post_load(void *opaque, int version_id)
     return 0;
 }
 
+static bool hpet_offset_needed(void *opaque)
+{
+    HPETState *s = opaque;
+
+    return hpet_enabled(s) && s->hpet_offset_saved;
+}
+
 static bool hpet_rtc_irq_level_needed(void *opaque)
 {
     HPETState *s = opaque;
@@ -285,6 +298,17 @@ static const VMStateDescription vmstate_hpet_rtc_irq_level = {
     }
 };
 
+static const VMStateDescription vmstate_hpet_offset = {
+    .name = "hpet/offset",
+    .version_id = 1,
+    .minimum_version_id = 1,
+    .needed = hpet_offset_needed,
+    .fields = (VMStateField[]) {
+        VMSTATE_UINT64(hpet_offset, HPETState),
+        VMSTATE_END_OF_LIST()
+    }
+};
+
 static const VMStateDescription vmstate_hpet_timer = {
     .name = "hpet_timer",
     .version_id = 1,
@@ -320,6 +344,7 @@ static const VMStateDescription vmstate_hpet = {
     },
     .subsections = (const VMStateDescription*[]) {
         &vmstate_hpet_rtc_irq_level,
+        &vmstate_hpet_offset,
         NULL
     }
 };
@@ -762,6 +787,7 @@ static Property hpet_device_properties[] = {
     DEFINE_PROP_UINT8("timers", HPETState, num_timers, HPET_MIN_TIMERS),
     DEFINE_PROP_BIT("msi", HPETState, flags, HPET_MSI_SUPPORT, false),
     DEFINE_PROP_UINT32(HPET_INTCAP, HPETState, intcap, 0),
+    DEFINE_PROP_BOOL("hpet-offset-saved", HPETState, hpet_offset_saved, true),
     DEFINE_PROP_END_OF_LIST(),
 };
 
diff --git a/include/hw/compat.h b/include/hw/compat.h
index 263de97..7f31850 100644
--- a/include/hw/compat.h
+++ b/include/hw/compat.h
@@ -2,7 +2,11 @@
 #define HW_COMPAT_H
 
 #define HW_COMPAT_2_11 \
-    /* empty */
+    {\
+        .driver   = "hpet",\
+        .property = "hpet-offset-saved",\
+        .value    = "false",\
+    },
 
 #define HW_COMPAT_2_10 \
     {\

Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru> wrote:
> HPET saves its state by calculating the current time and recovers timer
> offset using this calculated value. But these calculations include
> divisions and multiplications. Therefore the timer state cannot be recovered
> precise enough.
> This patch introduces saving of the original value of the offset to
> preserve the determinism of the timer.
>
> Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
> Signed-off-by: Maria Klimushenkova <maria.klimushenkova@ispras.ru>
> Reviewed-by: Juan Quintela <quintela@redhat.com>
>
> --
> v3: Added compat property for correct migration.

Yeap, Much better.

> @@ -762,6 +787,7 @@ static Property hpet_device_properties[] = {
>      DEFINE_PROP_UINT8("timers", HPETState, num_timers, HPET_MIN_TIMERS),
>      DEFINE_PROP_BIT("msi", HPETState, flags, HPET_MSI_SUPPORT, false),
>      DEFINE_PROP_UINT32(HPET_INTCAP, HPETState, intcap, 0),
> +    DEFINE_PROP_BOOL("hpet-offset-saved", HPETState, hpet_offset_saved, true),
>      DEFINE_PROP_END_OF_LIST(),
>  };

I am thinking about the name, but I can't suggest anything else that is
short and descriptive, so ....

Later, Juan.