[tenebrisalietum]

"Running inside a hypervisor" is not equivalent to the UEFI setting interrupt vectors to point to its own code.

Not sure how UEFI's "Compatibility Service Module" works exactly but if it acts like the original BIOS did, it's just a chunk of code that can be called, either by a program or be set as the destination for interrupts.

But there is no "VM exit" mechanism like there is in hypervisors. One thing a hypervisor does is intercept IO address accesses and does them on behalf of the client code (without the code knowing), don't think the CSM is doing this.

[zaarn]

To my knowledge, UEFI does sorta do hypervisor-y things when running a 16bit bootloader since the GPU needs to be operated from 64bit ( via it's UEFI driver) and keyboard and mouse need to be emulated via UEFI drivers, which are also 64bit. That is unless the device has a CSM module and can run natively 16bit code during CSM boot, which not all devices can do.