[PATCH] KVM: VMX: Optimize tscdeadline timer latency

[Wanpeng Li]

From: Wanpeng Li <wanpengli@xxxxxxxxxxx>

'Commit d0659d946be0 ("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). clockevents infrastruture 
can program minimum delay if hrtimer feeds a expiration in the past, 
we set delta_tsc to 1(which will be converted to 0 before vmentry) 
which can lead 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 <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
---
 arch/x86/kvm/vmx.c | 8 +++++++-
 arch/x86/kvm/x86.c | 1 +
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index e50beb7..d8b5110 100644
--- a/arch/x86/kvm/vmx.c
+++ b/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 = 1;
 
 	/* Convert to host delta tsc if tsc scaling is enabled */
 	if (vcpu->arch.tsc_scaling_ratio != kvm_default_tsc_scaling_ratio &&
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 22bd20f..d8abd96 100644
--- a/arch/x86/kvm/x86.c
+++ b/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);
-- 
2.7.4