KVM: VMX: Optimize tscdeadline timer latency

'Commit d0659d9 ("KVM: x86: add option to advance tscdeadline
hrtimer expiration")' advances the tscdeadline (the timer is emulated
by hrtimer) expiration in order that the latency which is incurred
by hypervisor (apic_timer_fn -> vmentry) can be avoided. This patch
adds the advance tscdeadline expiration support to which the tscdeadline
timer is emulated by VMX preemption timer to reduce the hypervisor
lantency (handle_preemption_timer -> vmentry). The guest can also
set an expiration that is very small (for example in Linux if an
hrtimer feeds a expiration in the past); in that case we set delta_tsc
to 0, leading to an immediately vmexit when delta_tsc is not bigger than
advance ns.

This patch can reduce ~63% latency (~4450 cycles to ~1660 cycles on
a haswell desktop) for kvm-unit-tests/tscdeadline_latency when testing
busy waits.

Cc: Paolo Bonzini <[email protected]>
Cc: Radim Krčmář <[email protected]>
Signed-off-by: Wanpeng Li <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: Wanpeng Li

arch/x86/kvm/vmx.c

@@ -12435,7 +12435,7 @@ static inline int u64_shl_div_u64(u64 a, unsigned int shift,
 static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc)
 {
 	struct vcpu_vmx *vmx;
-	u64 tscl, guest_tscl, delta_tsc;
+	u64 tscl, guest_tscl, delta_tsc, lapic_timer_advance_cycles;
 
 	if (kvm_mwait_in_guest(vcpu->kvm))
 		return -EOPNOTSUPP;
@@ -12444,6 +12444,12 @@ static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc)
 	tscl = rdtsc();
 	guest_tscl = kvm_read_l1_tsc(vcpu, tscl);
 	delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl;
+	lapic_timer_advance_cycles = nsec_to_cycles(vcpu, lapic_timer_advance_ns);
+
+	if (delta_tsc > lapic_timer_advance_cycles)
+		delta_tsc -= lapic_timer_advance_cycles;
+	else
+		delta_tsc = 0;
 
 	/* Convert to host delta tsc if tsc scaling is enabled */
 	if (vcpu->arch.tsc_scaling_ratio != kvm_default_tsc_scaling_ratio &&

arch/x86/kvm/x86.c

@@ -138,6 +138,7 @@ module_param(tsc_tolerance_ppm, uint, S_IRUGO | S_IWUSR);
 /* lapic timer advance (tscdeadline mode only) in nanoseconds */
 unsigned int __read_mostly lapic_timer_advance_ns = 0;
 module_param(lapic_timer_advance_ns, uint, S_IRUGO | S_IWUSR);
+EXPORT_SYMBOL_GPL(lapic_timer_advance_ns);
 
 static bool __read_mostly vector_hashing = true;
 module_param(vector_hashing, bool, S_IRUGO);