[PATCH 6/5] KVM: x86: fix periodic lapic timer with hrtimers

[Radim Krčmář]

I have only compile-tested it, but it should optimize the switch and
also fix two bugs.  The first one is major.
This needs that the deadline clearing in [5/5] is fixed.
---8<---
We must start the hrimer even if the expiration is already in the past,
otherwise the periodic timer would not rearm the hrtimer.

And computing next expiration of a period timer does not require current
time.  The period should be constant, so it is more precise to add the
period to the last expiration time.  This fixes a time difference
between hrtimer expiration and apic->lapic_timer.target_expiration.

Signed-off-by: Radim Krčmář <rkrc...@redhat.com>
---
 arch/x86/kvm/lapic.c | 38 +++++++++++++-------------------------
 1 file changed, 13 insertions(+), 25 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index f4734fe12803..6244988418be 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1350,19 +1350,19 @@ static void start_sw_tscdeadline(struct kvm_lapic *apic)
 
 static void start_sw_period(struct kvm_lapic *apic)
 {
-       ktime_t now;
-
-       now = apic->lapic_timer.timer.base->get_time();
-
        if (!apic->lapic_timer.period)
                return;
 
-       if (likely(ktime_after(apic->lapic_timer.target_expiration, now)))
-               hrtimer_start(&apic->lapic_timer.timer,
-                       apic->lapic_timer.target_expiration,
-                       HRTIMER_MODE_ABS_PINNED);
-       else
+       if (apic_lvtt_oneshot(apic) &&
+           ktime_after(apic->lapic_timer.target_expiration,
+                       apic->lapic_timer.timer.base->get_time())) {
                apic_timer_expired(apic);
+               return;
+       }
+
+       hrtimer_start(&apic->lapic_timer.timer,
+               apic->lapic_timer.target_expiration,
+               HRTIMER_MODE_ABS_PINNED);
 }
 
 static bool set_target_expiration(struct kvm_lapic *apic)
@@ -1414,10 +1414,11 @@ static bool set_target_expiration(struct kvm_lapic 
*apic)
 
 static void advance_periodic_target_expiration(struct kvm_lapic *apic)
 {
-       apic->lapic_timer.tscdeadline = kvm_read_l1_tsc(apic->vcpu, rdtsc()) +
+       apic->lapic_timer.tscdeadline +=
                nsec_to_cycles(apic->vcpu, apic->lapic_timer.period);
-       apic->lapic_timer.target_expiration = 
ktime_add_ns(apic->lapic_timer.timer.base->get_time(),
-               apic->lapic_timer.period);
+       apic->lapic_timer.target_expiration =
+               ktime_add_ns(apic->lapic_timer.target_expiration,
+                               apic->lapic_timer.period);
 }
 
 bool kvm_lapic_hv_timer_in_use(struct kvm_vcpu *vcpu)
@@ -1481,19 +1482,6 @@ void kvm_lapic_switch_to_hv_timer(struct kvm_vcpu *vcpu)
 
        WARN_ON(apic->lapic_timer.hv_timer_in_use);
 
-       if (apic_lvtt_period(apic)) {
-               ktime_t remaining, now;
-               u64 tscl = rdtsc();
-
-               now = apic->lapic_timer.timer.base->get_time();
-               remaining = hrtimer_get_remaining(&apic->lapic_timer.timer);
-               if (ktime_to_ns(remaining) < 0)
-                       remaining = ktime_set(0, 0);
-
-               apic->lapic_timer.tscdeadline = kvm_read_l1_tsc(apic->vcpu, 
tscl) +
-                       nsec_to_cycles(apic->vcpu, ktime_to_ns(remaining));
-               apic->lapic_timer.target_expiration = ktime_add_ns(now, 
ktime_to_ns(remaining));
-       }
        start_hv_timer(apic);
 }
 EXPORT_SYMBOL_GPL(kvm_lapic_switch_to_hv_timer);
-- 
2.10.1