Virtualization CPU Emulation

Question

I have a question about CPU virtualization from a virtual machine. I am not able to understand the difference between on-the-fly to native code translation and trap-and-emulate translation.

As far as I understand, in the first case suppose I emulate binary code from a different platform the code is converted to the equivalent x86 instruction if I have an x86 CPU. Now in the trap-and-emulate method the virtual machine receives the ISA call from the guest OS and translates it to the equivalent ISA call for the host OS.

Why do we need to translate from ISA to ISA? Suppose I am running an Ubuntu guest on a Windows host. The Ubuntu ISA call is different from the Windows ISA call? I understand that the Guest is not able to access System ISA on the host, only the monitor can do that. But why there is a need of conversion to the Host ISA? The ISA depends also on the operating system?

Answer 1

"On-the-fly to native" translation (often called JIT compilation/translation) is used when running code from one ISA on another ISA, such as running M68K code on an x86 CPU. It's in no way virtualization, but emulation.

Trap-and-emulate is a way to run "privileged" code in an unprivileged environment (example: running a kernel as an application). The way it works is that you start executing the privileged code, and once it tries to execute a privileged instruction (lidt in x86 for example), the host OS will issue a trap. In the handler for that trap, you could emulate that specific privileged instruction, and then let the guest kernel continue executing. The advantage of this is that you will reach close to native speeds for CPU emulation.

However, just emulating the ISA is only a "small" part of emulating a complete system. Emulating/virtualization of the MMU is much more complex to get right, and to get running fast.