"What, How, and Why" on Interrupt Window (or NMI Window) Exiting in Virtualization Technology

More recently, one of my colleagues asked me why there is a feature called "Interrupt Window exiting" in virtualization technology, and how it can be used by VMM? This blog is going to briefly describe its "what, how and why" .



WHAT, and HOW

"Interrupt Window Exiting" is one of VM exit reasons (#7 in Intel Technology). If “interrupt-window exiting” VM-execution control is set, this VM exit happens right after a VM entry and at the beginning of an any instruction:

• at which RFLAGS.IF = 1 (external interrupt is unmaksed) and,
• on which the interruptibility state of the guest would allow delivery of an interrupt (for example, not being blocked by STI or by MOV SS).

WHY
In a typical case, the VMM software wants to inject/deliver a (virtual) interrupt to its one of Guest VM at some point, but unfortunately the interruptibility state of its guest would NOT allow delivery of an interrupt at that moment (for example, since its guest RFLAGS.IF = 0). 

So, in order to deliver this interrupt, the VMM will need to poll and check the interruptibility state of the guest, once the interruptibility state of its guest allows delivery of an interrupt (A window is open), then VMM can deliver it at this moment. This is inefficient way to do so.

So, the problem is that -- How does a VMM get to know when its guest becomes interruptible? 

With this feature supported, a VMM is allowed to queue a virtual interrupt to its guest when the guest is not in an interruptible state. The VMM can just only set the “interrupt-window exiting” VM-execution control for that guest and depend on a VM exit to know when the guest becomes interruptible (and, therefore, when it can inject a virtual interrupt). The VMM can detect such VM exits by checking for the basic exit reason “interrupt-window”, if the value of exit reason is 7, then VMM knows it is right time to deliver a virtual interrupt to its specific guest.


Similarly, those also apply to "NMI window exiting" feature in Virtualization Technology.