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authorWanpeng Li <wanpengli@tencent.com>2018-05-29 14:53:17 +0800
committerPaolo Bonzini <pbonzini@redhat.com>2018-06-04 17:51:59 +0200
commitc5ce8235cffa00c207e24210329094d7634bb467 (patch)
tree9bc040bb9c249aa5792c06f869a1dedb51afa0db /arch/x86/kvm
parent71e9d9aeec7036ece9de8e51082f94d59c31be9a (diff)
KVM: VMX: Optimize tscdeadline timer latency
'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). 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 <pbonzini@redhat.com> Cc: Radim Krčmář <rkrcmar@redhat.com> Signed-off-by: Wanpeng Li <wanpengli@tencent.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'arch/x86/kvm')
-rw-r--r--arch/x86/kvm/vmx.c8
-rw-r--r--arch/x86/kvm/x86.c1
2 files changed, 8 insertions, 1 deletions
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index d205e9246f99..aff0f3ee6a1d 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 = 0;
/* 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 1d3dfc2c941d..93dd25d005a1 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);