Due to a offset between the clock used to generate the in-kernel
count_load_time (CLOCK_MONOTONIC) and the clock used for processing this
in userspace (vm_clock), reading back the output of PIT channel 2 via
port 0x61 was broken. One use cases that suffered from it was the CPU
frequency calibration of SeaBIOS, which also affected IDE/AHCI timeouts.
This fixes it by calibrating the offset between both clocks on
kvm_pit_get and adjusting the kernel value before saving it in the
userspace state. As the calibration only works while the vm_clock is
running, we cache the in-kernel state across stopped phases.
Signed-off-by: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
---
Should go into 1.1 as well if it's fine for master.
hw/kvm/i8254.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++----
1 files changed, 52 insertions(+), 5 deletions(-)
diff --git a/hw/kvm/i8254.c b/hw/kvm/i8254.c
index bb5fe07..04333d8 100644
--- a/hw/kvm/i8254.c
+++ b/hw/kvm/i8254.c
@@ -23,31 +23,63 @@
* THE SOFTWARE.
*/
#include "qemu-timer.h"
+#include "sysemu.h"
#include "hw/i8254.h"
#include "hw/i8254_internal.h"
#include "kvm.h"
#define KVM_PIT_REINJECT_BIT 0
+#define CALIBRATION_ROUNDS 3
+
typedef struct KVMPITState {
PITCommonState pit;
LostTickPolicy lost_tick_policy;
+ bool state_valid;
} KVMPITState;
-static void kvm_pit_get(PITCommonState *s)
+static int64_t abs64(int64_t v)
{
+ return v < 0 ? -v : v;
+}
+
+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;
+ int64_t offset, clock_offset;
+ struct timespec ts;
int i, ret;
+ if (s->state_valid) {
+ return;
+ }
+
+ /*
+ * Measure the delta between CLOCK_MONOTONIC, the base used for
+ * kvm_pit_channel_state::count_load_time, and vm_clock. Take the
+ * minimum of several samples to filter out scheduling noise.
+ */
+ clock_offset = LLONG_MAX;
+ for (i = 0; i < CALIBRATION_ROUNDS; i++) {
+ offset = qemu_get_clock_ns(vm_clock);
+ clock_gettime(CLOCK_MONOTONIC, &ts);
+ offset -= ts.tv_nsec;
+ offset -= (int64_t)ts.tv_sec * 1000000000;
+ if (abs64(offset) < abs64(clock_offset)) {
+ clock_offset = offset;
+ }
+ }
+
if (kvm_has_pit_state2()) {
ret = kvm_vm_ioctl(kvm_state, KVM_GET_PIT2, &kpit);
if (ret < 0) {
fprintf(stderr, "KVM_GET_PIT2 failed: %s\n", strerror(ret));
abort();
}
- s->channels[0].irq_disabled = kpit.flags & KVM_PIT_FLAGS_HPET_LEGACY;
+ pit->channels[0].irq_disabled = kpit.flags & KVM_PIT_FLAGS_HPET_LEGACY;
} else {
/*
* kvm_pit_state2 is superset of kvm_pit_state struct,
@@ -61,7 +93,7 @@ static void kvm_pit_get(PITCommonState *s)
}
for (i = 0; i < 3; i++) {
kchan = &kpit.channels[i];
- sc = &s->channels[i];
+ sc = &pit->channels[i];
sc->count = kchan->count;
sc->latched_count = kchan->latched_count;
sc->count_latched = kchan->count_latched;
@@ -74,10 +106,10 @@ static void kvm_pit_get(PITCommonState *s)
sc->mode = kchan->mode;
sc->bcd = kchan->bcd;
sc->gate = kchan->gate;
- sc->count_load_time = kchan->count_load_time;
+ sc->count_load_time = kchan->count_load_time + clock_offset;
}
- sc = &s->channels[0];
+ sc = &pit->channels[0];
sc->next_transition_time =
pit_get_next_transition_time(sc, sc->count_load_time);
}
@@ -173,6 +205,19 @@ static void kvm_pit_irq_control(void *opaque, int n, int enable)
kvm_pit_put(pit);
}
+static void kvm_pit_vm_state_change(void *opaque, int running,
+ RunState state)
+{
+ KVMPITState *s = opaque;
+
+ if (running) {
+ s->state_valid = false;
+ } else {
+ kvm_pit_get(&s->pit);
+ s->state_valid = true;
+ }
+}
+
static int kvm_pit_initfn(PITCommonState *pit)
{
KVMPITState *s = DO_UPCAST(KVMPITState, pit, pit);
@@ -215,6 +260,8 @@ static int kvm_pit_initfn(PITCommonState *pit)
qdev_init_gpio_in(&pit->dev.qdev, kvm_pit_irq_control, 1);
+ qemu_add_vm_change_state_handler(kvm_pit_vm_state_change, s);
+
return 0;
}
--
1.7.3.4
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