本文重点分析中断的处理流程。
(1)pic中断处理流程
为了一窥中断处理的具体流程，这里我们选择最简单模拟串口为例进行分析。qemu作为设备模拟器会模拟很多传统的设备，isa-serial就是其中之一。我们看下串口触发中断时候的调用栈：
#0 0x00005555557dd543 in kvm_set_irq (s=0x5555568f4440, irq=4, level=1) at /home/fang/code/qemu/accel/kvm/kvm-all.c:991
#1 0x0000555555881c0f in kvm_pic_set_irq (opaque=0x0, irq=4, level=1) at /home/fang/code/qemu/hw/i386/kvm/i8259.c:114
#2 0x00005555559cb0aa in qemu_set_irq (irq=0x5555577c9dc0, level=1) at hw/core/irq.c:45
#3 0x0000555555881fda in kvm_pc_gsi_handler (opaque=0x555556b61970, n=4, level=1) at /home/fang/code/qemu/hw/i386/kvm/ioapic.c:55
#4 0x00005555559cb0aa in qemu_set_irq (irq=0x555556b63660, level=1) at hw/core/irq.c:45
#5 0x00005555559c06e7 in qemu_irq_raise (irq=0x555556b63660) at /home/fang/code/qemu/include/hw/irq.h:16
#6 0x00005555559c09b3 in serial_update_irq (s=0x555557b77770) at hw/char/serial.c:145
#7 0x00005555559c138c in serial_ioport_write (opaque=0x555557b77770, addr=1, val=2, size=1) at hw/char/serial.c:404
可以看到qemu用户态有个函数kvm_set_irq，这个函数是用户态通知kvm内核态触发一个中断的入口。函数中通过调用 kvm_vm_ioctl注入一个中断，调用号是 KVM_IRQ_LINE（pic类型中断），入参是一个 kvm_irq_level 的数据结构（传入了irq编号和中断的电平信息）。模拟isa串口是个isa设备使用边沿触发，所以注入中断会调用2次这个函数前后2次电平相反。
int kvm_set_irq(KVMState *s, int irq, int level)
{
struct kvm_irq_level event;
int ret;
assert(kvm_async_interrupts_enabled());
event.level = level;
event.irq = irq;
ret = kvm_vm_ioctl(s, s->irq_set_ioctl, &event);
if (ret < 0) {
perror("kvm_set_irq");
abort();
}
return (s->irq_set_ioctl == KVM_IRQ_LINE) ? 1 : event.status;
}
这个ioctl在内核的处理对应到下面这段代码，pic类型中断进而会调用到kvm_vm_ioctl_irq_line函数。
kvm_vm_ioctl
{
......
#ifdef __KVM_HAVE_IRQ_LINE
case KVM_IRQ_LINE_STATUS:
case KVM_IRQ_LINE: { /* 处理pic上产生的中断 */
struct kvm_irq_level irq_event;
r = -EFAULT;
if (copy_from_user(&irq_event, argp, sizeof(irq_event)))
goto out;
r = kvm_vm_ioctl_irq_line(kvm, &irq_event,
ioctl == KVM_IRQ_LINE_STATUS);
if (r)
goto out;
r = -EFAULT;
if (ioctl == KVM_IRQ_LINE_STATUS) {
if (copy_to_user(argp, &irq_event, sizeof(irq_event)))
goto out;
}
r = 0;
break;
}
#endif
#ifdef CONFIG_HAVE_KVM_IRQ_ROUTING /* 处理ioapic的中断 */
case KVM_SET_GSI_ROUTING: {
struct kvm_irq_routing routing;
struct kvm_irq_routing __user *urouting;
struct kvm_irq_routing_entry *entries = NULL;
r = -EFAULT;
if (copy_from_user(&routing, argp, sizeof(routing)))
goto out;
r = -EINVAL;
if (!kvm_arch_can_set_irq_routing(kvm))
goto out;
if (routing.nr > KVM_MAX_IRQ_ROUTES)
goto out;
if (routing.flags)
goto out;
if (routing.nr) {
r = -ENOMEM;
entries = vmalloc(routing.nr * sizeof(*entries));
if (!entries)
goto out;
r = -EFAULT;
urouting = argp;
if (copy_from_user(entries, urouting->entries,
routing.nr * sizeof(*entries)))
goto out_free_irq_routing;
}
r = kvm_set_irq_routing(kvm, entries, routing.nr,
routing.flags);
out_free_irq_routing:
vfree(entries);
break;
}
......
}
kvm_vm_ioctl_irq_line函数中会进一步调用内核态的kvm_set_irq函数（用户态用同名函数额），这个函数是整个中断处理的入口：
int kvm_vm_ioctl_irq_line(struct kvm *kvm, struct kvm_irq_level *irq_event,
bool line_status)
{
if (!irqchip_in_kernel(kvm))
return -ENXIO;
irq_event->status = kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID,
irq_event->irq, irq_event->level,
line_status);
return 0;
}
kvm_set_irq函数的入参有5个，kvm代表某个特性的的虚拟机，irq_source_id可以是KVM_USERSPACE_IRQ_SOURCE_ID或者KVM_IRQFD_RESAMPLE_IRQ_SOURCE_ID(这个是irqfd这个我们这里不讨论)，irq是传入的设备irq号，对于串口来说第一个port的irq=4而且irq=gsi，level代表电平。kvm_irq_map函数会获取改gsi索引上注册的中断路由项(kvm_kernel_irq_routing_entry)，while循环中会挨个调用每个中断路由项上的set方法触发中,在guest中会忽略没有实现的芯片类型发送的中断消息。
对于PIC而言，set函数对应于kvm_set_pic_irq函数，对于IOAPIC而言set函数对应于kvm_set_ioapic_irq（不同的chip不一样额）。对于串口而言，我们会进一步调用kvm_pic_set_irq来处理中断。
/*
* Return value:
* < 0 Interrupt was ignored (masked or not delivered for other reasons)
* = 0 Interrupt was coalesced (previous irq is still pending)
* > 0 Number of CPUs interrupt was delivered to
*/
int kvm_set_irq(struct kvm *kvm, int irq_source_id, u32 irq, int level,
bool line_status)
{
struct kvm_kernel_irq_routing_entry irq_set[KVM_NR_IRQCHIPS];
int ret = -1, i, idx;
trace_kvm_set_irq(irq, level, irq_source_id);
/* Not possible to detect if the guest uses the PIC or the
* IOAPIC. So set the bit in both. The guest will ignore
* writes to the unused one.
*/
idx = srcu_read_lock(&kvm->irq_srcu);
i = kvm_irq_map_gsi(kvm, irq_set, irq);
srcu_read_unlock(&kvm->irq_srcu, idx);
/* 依次调用同一个gsi上的所有芯片的set方法 */
while (i--) {
int r;
r = irq_set[i].set(&irq_set[i], kvm, irq_source_id, level,
line_status);
if (r < 0)
continue;
ret = r + ((ret < 0) ? 0 : ret);
}
return ret;
}
/* 查询出此gsi号上对应的所有的“中断路由项” */
int kvm_irq_map_gsi(struct kvm *kvm,
struct kvm_kernel_irq_routing_entry *entries, int gsi)
{
struct kvm_irq_routing_table *irq_rt;
struct kvm_kernel_irq_routing_entry *e;
int n = 0;
irq_rt = srcu_dereference_check(kvm->irq_routing, &kvm->irq_srcu,
lockdep_is_held(&kvm->irq_lock));
if (irq_rt && gsi < irq_rt->nr_rt_entries) {
hlist_for_each_entry(e, &irq_rt->map[gsi], link) { /* 遍历此gsi对应的中断路由项 */
entries[n] = *e;
++n;
}
}
return n;
}
kvm_pic_set_irq 函数中，根据传入的irq编号check下原先的irq_state将本次的level与上次的irq_state进行逻辑“异或”判断是否发生电平跳变从而进行边沿检测（pic_set_irq1）。如果是的话设置IRR对应的bit，然后调用和pic_update_irq更新pic相关的寄存器并唤醒vcpu注入中断。
int kvm_pic_set_irq(struct kvm_pic *s, int irq, int irq_source_id, int level)
{
int ret, irq_level;
BUG_ON(irq < 0 || irq >= PIC_NUM_PINS);
pic_lock(s);
/* irq_level = 1表示该irq引脚有电平跳变，出发中断 */
irq_level = __kvm_irq_line_state(&s->irq_states[irq],
irq_source_id, level);
/* 一个pic最多8个irq */
ret = pic_set_irq1(&s->pics[irq >> 3], irq & 7, irq_level);
pic_update_irq(s);
trace_kvm_pic_set_irq(irq >> 3, irq & 7, s->pics[irq >> 3].elcr,
s->pics[irq >> 3].imr, ret == 0);
pic_unlock(s);
return ret;
}
最后的最后，pic_unlock函数中在如果wakeup为true（又中断产生时）的时候会遍历每个vcpu，在满足条件的情况下调用kvm_make_request为vcpu注入中断，然后kick每个vcpu。
static void pic_unlock(struct kvm_pic *s)
__releases(&s->lock)
{
bool wakeup = s->wakeup_needed;
struct kvm_vcpu *vcpu;
int i;
s->wakeup_needed = false;
spin_unlock(&s->lock);
if (wakeup) { /* wakeup在pic_update_irq中被更新 */
kvm_for_each_vcpu(i, vcpu, s->kvm) {
if (kvm_apic_accept_pic_intr(vcpu)) {
/* 中断注入会在kvm_enter_guest时候执行 */
kvm_make_request(KVM_REQ_EVENT, vcpu);
kvm_vcpu_kick(vcpu);
return;
}
}
}
}