Linux： Intel-Virtualization Technology(VT)下的中断

这也是以前的一篇手记，由于涉及到公司的一些资料，摘录并整理。只是一个简单的流程说明，详细的知识点还请参考相关手册。

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先来张图热个身。

在VT环境下，当中断产生时，有两种场景：在host上下文和guest上下文。 host收到中断后，可以自己处理，也可以将此中断重新路由给Guest；Guest上下文的中断，可以配置为直接路由给Guest，或配置为路由给host。所以需要host和guest间有统一的中断配置说明。

host下收到中断的情况：
如果中断是需要host处理，则host直接处理；如果此中断需要由guest处理，利用虚拟中断机制传递中断。
根据为Guest的vector设置VM-entry interruption-information field；则在vm-entry时，会触发对应的vector中断给guest。

VM-entry interruption-information field (32 bits)的结构描述如下：

VMX operation allows injecting interruptions to a guest virtual machine through the use of VM-entry interrupt-information field in VMCS. Injectable interruptions include external interrupts, NMI, processor exceptions, software generated interrupts, and software traps. If the interrupt-information field indicates a valid interrupt, exception or trap event upon the next VM entry; the processor will use the information in the field to vector a virtual interruption through the guest IDT after all guest state and MSRs are loaded. Delivery through the guest IDT emulates vectoring in non-VMX operation by doing the normal privilege checks and pushing appropriate entries to the guest stack (entries may include RFLAGS, EIP and exception error code). A VMM with host control of NMI and external interrupts can use the event-injection facility to
forward virtual interruptions to various guest virtual machines.

guest下收到中断的情况：
可通过exception bitmap和External-interrupt exiting来控制异常和中断的流向，见下面的描述。

The exception bitmap is a 32-bit field that contains one bit for each exception. When an exception occurs, its vector is used to select a bit in this field. If the bit is 1, the exception causes a VM exit. If the bit is 0, the exception is delivered normally through the IDT, using the descriptor corresponding to the exception’s vector.

External-interrupt exiting：If this control is 1, external interrupts cause VM exits. Otherwise, they are delivered normally through the guest interrupt-descriptor table (IDT). If this control is 1, the value of RFLAGS.IF does not affect interrupt blocking.

如果中断需要路由给host，则中断会触发vm-exit，退出的理由是：
External interrupt. An external interrupt arrived and the “external-interrupt exiting” VM-execution control was 1.

如果中断没有配置为退出vm，则直接路由给guest的IDT。

host和guest之间也可互相发送软中断，作为2者间信息交互的触发手段
host–>guest: 如前所述，利用VM-entry interruption-information field等控制位。
guest–>host: 利用vmcall

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下图是当时画的中断重定向图。Line经由APIC重定向到IDT的vector；vector经由IDT重定向到不同的中断门；在经由中断门指向Linux的Irq号以及处理函数。