Patchwork [uq/master] kvm: i8254: Finish time conversion fix

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Submitter Jan Kiszka
Date Aug. 13, 2012, 6:18 p.m.
Message ID <502944F9.9070200@siemens.com>
Download mbox | patch
Permalink /patch/177017/
State New
Headers show

Comments

Jan Kiszka - Aug. 13, 2012, 6:18 p.m.
0cdd3d1444 fixed reading back the counter load time from the kernel
while assuming the kernel would always update its load time on writing
the state. That is only true for channel 1, and so pit_get_channel_info
returned wrong output pin states for high counter values.

Fix this by applying the offset also on kvm_pit_put. For this purpose,
we cache the clock offset in KVMPITState, only updating it on VM state
changes or when we write the state while the VM is stopped.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---
 hw/kvm/i8254.c |   52 ++++++++++++++++++++++++++++++++++------------------
 1 files changed, 34 insertions(+), 18 deletions(-)
Michael Tokarev - Aug. 13, 2012, 6:40 p.m.
On 13.08.2012 22:18, Jan Kiszka wrote:
> 0cdd3d1444 fixed reading back the counter load time from the kernel
> while assuming the kernel would always update its load time on writing
> the state. That is only true for channel 1, and so pit_get_channel_info
> returned wrong output pin states for high counter values.
> 
> Fix this by applying the offset also on kvm_pit_put. For this purpose,
> we cache the clock offset in KVMPITState, only updating it on VM state
> changes or when we write the state while the VM is stopped.

Wug.  The fix (consisting of two halves) appears to be quite messy.
Is it a (temporary) workaround or a real solution?

And yes, this second half fixes the reported issue with grub timekeeping,
and should fix the seabios problem as well (so it shouldn't be necessary
to mess with timekeeping in seabios anymore).

Thank you Jan!

/mjt
Jan Kiszka - Aug. 14, 2012, 7:41 a.m.
On 2012-08-13 20:40, Michael Tokarev wrote:
> On 13.08.2012 22:18, Jan Kiszka wrote:
>> 0cdd3d1444 fixed reading back the counter load time from the kernel
>> while assuming the kernel would always update its load time on writing
>> the state. That is only true for channel 1, and so pit_get_channel_info
>> returned wrong output pin states for high counter values.
>>
>> Fix this by applying the offset also on kvm_pit_put. For this purpose,
>> we cache the clock offset in KVMPITState, only updating it on VM state
>> changes or when we write the state while the VM is stopped.
> 
> Wug.  The fix (consisting of two halves) appears to be quite messy.

I will split it up into offset caching and application to kvm_pit_put.

> Is it a (temporary) workaround or a real solution?

No, this is the real solution. It may look complex, but it is required
due to the different time bases of the in-kernel PIT and QEMU's vmclock.
We didn't care about this in qemu-kvm in the past, but upstream now
actually supports migration between in-kernel and user space models, and
it also supports the PC speaker with the in-kernel PIT enabled.

> 
> And yes, this second half fixes the reported issue with grub timekeeping,
> and should fix the seabios problem as well (so it shouldn't be necessary
> to mess with timekeeping in seabios anymore).

Thanks, great to hear!

Jan

Patch

diff --git a/hw/kvm/i8254.c b/hw/kvm/i8254.c
index c5d3711..53d13e3 100644
--- a/hw/kvm/i8254.c
+++ b/hw/kvm/i8254.c
@@ -35,7 +35,8 @@ 
 typedef struct KVMPITState {
     PITCommonState pit;
     LostTickPolicy lost_tick_policy;
-    bool state_valid;
+    bool vm_stopped;
+    int64_t kernel_clock_offset;
 } KVMPITState;
 
 static int64_t abs64(int64_t v)
@@ -43,19 +44,11 @@  static int64_t abs64(int64_t v)
     return v < 0 ? -v : v;
 }
 
-static void kvm_pit_get(PITCommonState *pit)
+static void kvm_pit_update_clock_offset(KVMPITState *s)
 {
-    KVMPITState *s = DO_UPCAST(KVMPITState, pit, pit);
-    struct kvm_pit_state2 kpit;
-    struct kvm_pit_channel_state *kchan;
-    struct PITChannelState *sc;
     int64_t offset, clock_offset;
     struct timespec ts;
-    int i, ret;
-
-    if (s->state_valid) {
-        return;
-    }
+    int i;
 
     /*
      * Measure the delta between CLOCK_MONOTONIC, the base used for
@@ -72,6 +65,21 @@  static void kvm_pit_get(PITCommonState *pit)
             clock_offset = offset;
         }
     }
+    s->kernel_clock_offset = clock_offset;
+}
+
+static void kvm_pit_get(PITCommonState *pit)
+{
+    KVMPITState *s = DO_UPCAST(KVMPITState, pit, pit);
+    struct kvm_pit_state2 kpit;
+    struct kvm_pit_channel_state *kchan;
+    struct PITChannelState *sc;
+    int i, ret;
+
+    /* No need to re-read the state if VM is stopped. */
+    if (s->vm_stopped) {
+        return;
+    }
 
     if (kvm_has_pit_state2()) {
         ret = kvm_vm_ioctl(kvm_state, KVM_GET_PIT2, &kpit);
@@ -106,7 +114,7 @@  static void kvm_pit_get(PITCommonState *pit)
         sc->mode = kchan->mode;
         sc->bcd = kchan->bcd;
         sc->gate = kchan->gate;
-        sc->count_load_time = kchan->count_load_time + clock_offset;
+        sc->count_load_time = kchan->count_load_time + s->kernel_clock_offset;
     }
 
     sc = &pit->channels[0];
@@ -114,17 +122,23 @@  static void kvm_pit_get(PITCommonState *pit)
         pit_get_next_transition_time(sc, sc->count_load_time);
 }
 
-static void kvm_pit_put(PITCommonState *s)
+static void kvm_pit_put(PITCommonState *pit)
 {
+    KVMPITState *s = DO_UPCAST(KVMPITState, pit, pit);
     struct kvm_pit_state2 kpit;
     struct kvm_pit_channel_state *kchan;
     struct PITChannelState *sc;
     int i, ret;
 
-    kpit.flags = s->channels[0].irq_disabled ? KVM_PIT_FLAGS_HPET_LEGACY : 0;
+    /* The offset keeps changing as long as the VM is stopped. */
+    if (s->vm_stopped) {
+        kvm_pit_update_clock_offset(s);
+    }
+
+    kpit.flags = pit->channels[0].irq_disabled ? KVM_PIT_FLAGS_HPET_LEGACY : 0;
     for (i = 0; i < 3; i++) {
         kchan = &kpit.channels[i];
-        sc = &s->channels[i];
+        sc = &pit->channels[i];
         kchan->count = sc->count;
         kchan->latched_count = sc->latched_count;
         kchan->count_latched = sc->count_latched;
@@ -137,7 +151,7 @@  static void kvm_pit_put(PITCommonState *s)
         kchan->mode = sc->mode;
         kchan->bcd = sc->bcd;
         kchan->gate = sc->gate;
-        kchan->count_load_time = sc->count_load_time;
+        kchan->count_load_time = sc->count_load_time - s->kernel_clock_offset;
     }
 
     ret = kvm_vm_ioctl(kvm_state,
@@ -211,10 +225,12 @@  static void kvm_pit_vm_state_change(void *opaque, int running,
     KVMPITState *s = opaque;
 
     if (running) {
-        s->state_valid = false;
+        kvm_pit_update_clock_offset(s);
+        s->vm_stopped = false;
     } else {
+        kvm_pit_update_clock_offset(s);
         kvm_pit_get(&s->pit);
-        s->state_valid = true;
+        s->vm_stopped = true;
     }
 }