Interrupts (x86_64)

Interrupts are how devices request service from the guest drivers. This page explores the details of interrupt routing from the perspective of CrosVM.

Critical acronyms

This subject area uses a lot of acronyms:

  • IRQ: Interrupt ReQuest
  • ISR: Interrupt Service Routine
  • EOI: End Of Interrupt
  • MSI: message signaled interrupts. In this document, synonymous with MSI-X.
  • MSI-X: message signaled interrupts - extended
  • LAPIC: local APIC
  • APIC: Advanced Programmable Interrupt Controller (successor to the legacy PIC)
  • IOAPIC: IO APIC (has physical interrupt lines, which it responds to by triggering an MSI directed to the LAPIC).
  • PIC: Programmable Interrupt Controller (the "legacy PIC" / Intel 8259 chip).

Interrupts come in two flavors

Interrupts on x86_64 in CrosVM come in two primary flavors: legacy and MSI-X. In this document, MSI is used to refer to the concept of message signaled interrupts, but it always refers to interrupts sent via MSI-X because that is what CrosVM uses.

Legacy interrupts (INTx)

These interrupts are traditionally delivered via dedicated signal lines to PICs and/or the IOAPIC. Older devices, especially those that are used during early boot, often rely on these types of interrupts. These typically are the first 24 GSIs, and are serviced either by the PIC (during very early boot), or by the IOAPIC (after it is activated & the PIC is switched off).

Background on EOI

The purpose of EOI is rooted in how legacy interrupt lines are shared. If two devices D1 and D2 share a line L, D2 has no guarantee that it will be serviced when L is asserted. After receiving EOI, D2 has to check whether it was serviced, and if it was not, to re-assert L. An example of how this occurs is if D2 requests service while D1 is already being serviced. In that case, the line has to be reasserted otherwise D2 won't be serviced.

Because interrupt lines to the IOAPIC can be shared by multiple devices, EOI is critical for devices to figure out whether they were serviced in response to sending the IRQ, or whether the IRQ needs to be resent. The operating principles mean that sending extra EOIs to a legacy device is perfectly safe, because they could be due to another device on the same line receiving service, and so devices must be tolerant of such "extra" (from their perspective) EOIs.

These "extra" EOIs come from the fact that EOI is often a broadcast message that goes to all legacy devices. Broadcast is required because interrupt lines can be routed through the two 8259 PICs via cascade before they reach the CPU, broadcast to both PICs (and attached devices) is the only way to ensure EOI reaches the device that was serviced.

EOI in CrosVM

When the guest's ISR completes and signals EOI, the CrosVM irqchip implementation is responsible for propagating EOI to the device backends. EOI is delivered to the devices via their resample event. Devices are then responsible for listening to that resample event, and checking their internal state to see if they received service. If the device wasn't serviced, it must then reassert the IRQ.


MSIs do not use dedicated signal lines; instead, they are "messages" which are sent on the system bus. The LAPIC(s) receive these messages, and inject the interrupt into the VCPU (where injection means: jump to ISR).

About EOI

EOI is not meaningful for MSIs because lines are never shared. No devices using MSI will listen for the EOI event, and the irqchip will not signal it.

The fundamental deception on x86_64: there are no legacy interrupts (usually)

After very early boot, the PIC is switched off and legacy interrupts somewhat cease to be legacy. Instead of being handled by the PIC, legacy interrupts are handled by the IOAPIC, and all the IOAPIC does is convert them into MSIs; in other words, from the perspective of CrosVM & the guest VCPUs, after early boot, every interrupt is a MSI.

Interrupt handling irqchip specifics

Each IrqChip can handle interrupts differently. Often these differences are because the underlying hypervisors will have different interrupt features such as KVM's irqfds. Generally a hypervisor has three choices for implementing an irqchip:

  • Fully in kernel: all of the irqchip (LAPIC & IOAPIC) are implemented in the kernel portion of the hypervisor.
  • Split: the performance critical part of the irqchip (LAPIC) is implemented in the kernel, but the IOAPIC is implemented by the VMM.
  • Userspace: here, the entire irqchip is implemented in the VMM. This is generally slower and not commonly used.

Below, we describe the rough flow for interrupts in virtio devices for each of the chip types. We limit ourselves to virtio devices becauseas these are the performance critical devices in CrosVM.

Kernel mode IRQ chip (w/ irqfd support)


  1. Device wants service, so it signals an Event object.
  2. The Event object is registered with the hypervisor, so the hypervisor immediately routes the IRQ to a LAPIC so a VCPU can be interrupted.
  3. The LAPIC interrupts the VCPU, which jumps to the kernel's ISR (interrupt service routine).
  4. The ISR runs.

Legacy interrupts

These are handled similarly to MSIs, except the kernel mode IOAPIC is what initially picks up the event, rather than the LAPIC.

Split IRQ chip (w/ irqfd support)

This is the same as the kernel mode case.

Split IRQ chip (no irqfd kernel support)


  1. Device wants service, so it signals an Event object.
  2. The Eventobject is attached to the IrqChip in CrosVM. An interrupt handling thread wakes up from the Event signal.
  3. The IrqChip resets the Event.
  4. The IrqChip asserts the interrupt to the LAPIC in the kernel via an ioctl (or equivalent).
  5. The LAPIC interrupts the VCPU, which jumps to the kernel’s ISR (interrupt service routine).
  6. The ISR runs, and on completion sends EOI (end of interrupt). In CrosVM, this is called the resample event.
  7. EOI is sent.

Legacy interrupts

This introduces an additional Event object in the interrupt path, since the IRQ pin itself is an Event, and the MSI is also an Event. These interrupts are processed twice by the IRQ handler: once as a legacy IOAPIC event, and a second time as an MSI.

Userspace IRQ chip

This chip is not widely used in production. Contributions to fill in this section are welcome.