Introduction to VT-x Posted-interrupt介绍了原理，本文将结合KVM代码，介绍VT-x Posted Interrupt的实现细节。

source code:
https://elixir.bootlin.com/linux/v5.14/source

vmx_deliver_posted_interrupt

假设guest运行在x2APIC mode下，当source vCPU写ICR寄存器时，会发生VM Exit。

KVM的函数调用链如下:

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kvm_emulate_wrmsr
	kvm_set_msr
		kvm_set_msr_ignored_check
			__kvm_set_msr
				static_call(kvm_x86_set_msr)[vmx_set_msr]
					kvm_set_msr_common
						kvm_x2apic_msr_write
							kvm_lapic_reg_write
								kvm_apic_send_ipi
									kvm_irq_delivery_to_apic

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kvm_irq_delivery_to_apic
	kvm_irq_delivery_to_apic_fast
		kvm_apic_set_irq
			__apic_accept_irq
				kvm_x86_deliver_posted_interrupt[vmx_deliver_posted_interrupt]
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/*
 * Send interrupt to vcpu via posted interrupt way.
 * 1. If target vcpu is running(non-root mode), send posted interrupt
 * notification to vcpu and hardware will sync PIR to vIRR atomically.
 * 2. If target vcpu isn't running(root mode), kick it to pick up the
 * interrupt from PIR in next vmentry.
 */
static int vmx_deliver_posted_interrupt(struct kvm_vcpu *vcpu, int vector)
{
	struct vcpu_vmx *vmx = to_vmx(vcpu);
	int r;

	...

	if (!vcpu->arch.apicv_active)
		return -1;

	if (pi_test_and_set_pir(vector, &vmx->pi_desc))
		return 0;

	/* If a previous notification has sent the IPI, nothing to do.  */
	if (pi_test_and_set_on(&vmx->pi_desc))
		return 0;

	if (vcpu != kvm_get_running_vcpu() &&
	    !kvm_vcpu_trigger_posted_interrupt(vcpu, false))
		// Kick a sleeping VCPU, or a guest VCPU in guest mode, into host kernel mode.
		kvm_vcpu_kick(vcpu);

	return 0;
}

static inline bool kvm_vcpu_trigger_posted_interrupt(struct kvm_vcpu *vcpu,
						     bool nested)
{
#ifdef CONFIG_SMP
	int pi_vec = POSTED_INTR_VECTOR;

	if (vcpu->mode == IN_GUEST_MODE) {
		// 给vCPU所在的physical CPU发送一个Notification Event
		apic->send_IPI_mask(get_cpu_mask(vcpu->cpu), pi_vec);
		return true;
	}
#endif
	return false;
}

If target vcpu isn’t running(root mode), kick it(kvm_vcpu_kick(vcpu)) to pick up the interrupt from PIR in next vmentry.

vmx_sync_pir_to_irr

pick up the interrupt from PIR in the next vmentry这一步的代码解析如下:

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int vcpu_enter_guest(struct kvm_vcpu *vcpu)
{
	...

	/*
	 * This handles the case where a posted interrupt was
	 * notified with kvm_vcpu_kick.
	 */
	if (kvm_lapic_enabled(vcpu) && vcpu->arch.apicv_active)
		// vmx_sync_pir_to_irr(vcpu) is called
		static_call(kvm_x86_sync_pir_to_irr)(vcpu);
	...
}
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static int vmx_sync_pir_to_irr(struct kvm_vcpu *vcpu)
{
	struct vcpu_vmx *vmx = to_vmx(vcpu);
	int max_irr;

	if (pi_test_on(&vmx->pi_desc)) {
		pi_clear_on(&vmx->pi_desc);
		kvm_apic_update_irr(vcpu, vmx->pi_desc.pir, &max_irr);
		kvm_make_request(KVM_REQ_EVENT, vcpu);
	} else {
		max_irr = kvm_lapic_find_highest_irr(vcpu);
	}
	vmx_set_rvi(max_irr);
	return max_irr;
}

kvm_apic_update_irr
	__kvm_apic_update_irr

bool __kvm_apic_update_irr(u32 *pir, void *regs, int *max_irr)
{
	u32 i, vec;
	u32 pir_val, irr_val, prev_irr_val;
	int max_updated_irr;

	max_updated_irr = -1;
	*max_irr = -1;

	for (i = vec = 0; i <= 7; i++, vec += 32) {
		pir_val = READ_ONCE(pir[i]);
		irr_val = *((u32 *)(regs + APIC_IRR + i * 0x10));
		if (pir_val) {
			prev_irr_val = irr_val;
			irr_val |= xchg(&pir[i], 0);
			*((u32 *)(regs + APIC_IRR + i * 0x10)) = irr_val;
			if (prev_irr_val != irr_val) {
				max_updated_irr =
					__fls(irr_val ^ prev_irr_val) + vec;
			}
		}
		if (irr_val)
			*max_irr = __fls(irr_val) + vec;
	}

	return ((max_updated_irr != -1) &&
		(max_updated_irr == *max_irr));
}

一言以蔽之: vmx_sync_pir_to_irr所做的工作就是下图绿框中标注的步骤。区别在于：如果target vcpu is running(non-root mode)，上述步骤由硬件来完成；否则，需要调用vmx_sync_pir_to_irr由软件来完成。

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