KVM: x86: add option to advance tscdeadline hrtimer expiration

For the hrtimer which emulates the tscdeadline timer in the guest,
add an option to advance expiration, and busy spin on VM-entry waiting
for the actual expiration time to elapse.

This allows achieving low latencies in cyclictest (or any scenario
which requires strict timing regarding timer expiration).

Reduces average cyclictest latency from 12us to 8us
on Core i5 desktop.

Note: this option requires tuning to find the appropriate value
for a particular hardware/guest combination. One method is to measure the
average delay between apic_timer_fn and VM-entry.
Another method is to start with 1000ns, and increase the value
in say 500ns increments until avg cyclictest numbers stop decreasing.

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

1 files changed, 3 insertions, 0 deletions

diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index c674fce53cf90..7054437944cda 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -14,6 +14,7 @@ struct kvm_timer {
 	u32 timer_mode;
 	u32 timer_mode_mask;
 	u64 tscdeadline;
+	u64 expired_tscdeadline;
 	atomic_t pending;			/* accumulated triggered timers */
 };
 
@@ -170,4 +171,6 @@ static inline bool kvm_apic_has_events(struct kvm_vcpu *vcpu)
 
 bool kvm_apic_pending_eoi(struct kvm_vcpu *vcpu, int vector);
 
+void wait_lapic_expire(struct kvm_vcpu *vcpu);
+
 #endif