本文参考的内核版本为v6.0。

Motivation

virtual IPI fastpath

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vmx_vcpu_run
└── vmx_exit_handlers_fastpath
    └── handle_fastpath_set_msr_irqoff
        └── handle_fastpath_set_x2apic_icr_irqoff
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/*
 * The fast path for frequent and performance sensitive wrmsr emulation,
 * i.e. the sending of IPI, sending IPI early in the VM-Exit flow reduces
 * the latency of virtual IPI by avoiding the expensive bits of transitioning
 * from guest to host, e.g. reacquiring KVM's SRCU lock. In contrast to the
 * other cases which must be called after interrupts are enabled on the host.
 */
static int handle_fastpath_set_x2apic_icr_irqoff(struct kvm_vcpu *vcpu, u64 data)
{
	if (!lapic_in_kernel(vcpu) || !apic_x2apic_mode(vcpu->arch.apic))
		return 1;

	if (((data & APIC_SHORT_MASK) == APIC_DEST_NOSHORT) &&
	    ((data & APIC_DEST_MASK) == APIC_DEST_PHYSICAL) &&
	    ((data & APIC_MODE_MASK) == APIC_DM_FIXED) &&
	    ((u32)(data >> 32) != X2APIC_BROADCAST))
		return kvm_x2apic_icr_write(vcpu->arch.apic, data);

	return 1;
}

virtual TSC-Deadline timer fastpath

KVM: X86: TSCDEADLINE MSR emulation fastpath

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vmx_vcpu_run
└── vmx_exit_handlers_fastpath
    └── handle_fastpath_set_msr_irqoff
        └── handle_fastpath_set_tscdeadline
            └── kvm_set_lapic_tscdeadline_msr

KVM: VMX: Handle preemption timer fastpath
该patch优化的是使用preemption timer模拟lapic timer的case。

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vmx_vcpu_run
└── vmx_exit_handlers_fastpath
    └── handle_fastpath_preemption_timer

参考资料:

  1. KVM Latency and Scalability Performance Tuning
  2. KVM: VMX: FIXED+PHYSICAL mode single target IPI fastpath
  3. KVM: VMX: Tscdeadline timer emulation fastpath