KVM: VMX: Optimize tscdeadline timer latency (c5ce8235) · Commits · 戴 / test

'Commit ("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>

arch/x86/kvm/vmx.c

+7 −1

Original line number	Diff line number	Diff line
		@@ -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

+1 −0

Original line number	Diff line number	Diff line
		@@ -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);

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