From c5ce8235cffa00c207e24210329094d7634bb467 Mon Sep 17 00:00:00 2001 From: Wanpeng Li Date: Tue, 29 May 2018 14:53:17 +0800 Subject: [PATCH] KVM: VMX: Optimize tscdeadline timer latency MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit '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 Cc: Radim Krčmář Signed-off-by: Wanpeng Li Signed-off-by: Paolo Bonzini --- 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 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); -- 2.45.2