Emulating an x86 in a JVM and getting speeds that are in any way decent sounds just about impossible, but this chapter walked through a lot of details on how they made it work.
I'm still not too clear on the difference between an emulator and a VM. I wish the author would have clarified that a little. I wish the author would have put the "Ultimate Flexibility" and "Ultimate Security" sections up front, because the whole time that I was reading the chapter, I was wondering what the big benefits are. Even after reading those sections, though, I'm still not clear on the emulator's advantages over a VM. One thing that they were touting as a feature was being able to snapshot the system in just about any state. However, I know that the infrastructure team at my company does something similar with VMs all the time. I'm guessing that the difference is that the VM snapshots don't capture active processes but the emulator snapshots do. Pretty cool features, though I'm still not sold on the practicality of the whole system.
The fact that this emulator would be running in the JVM's sandbox seemed like it would be a major problem because the JVM would restrict many operations that I imagine the emulator would try to perform. Some of these problems were mentioned in the Xen chapter on why hardware or software support was necessary for the hypervisors to work correctly and trap restricted operating system calls...though as I'm writing this I'm realizing that the problem there was calls failing silently without the hypervisor being able to trap them, but the emulator is going to see every single instruction. I'm still not sure how the emulator would be able to handle requests for access that the JVM prevents the emulator from carrying out.
Although I don't typically work in Java on a day to day basis, I found all of the low-level optimization techniques interesting. However, from a higher level design and architecture point of view, I didn't take away as much from this chapter. I did become much more familiar with the JVM, class loaders, etc., but that wasn't quite what I was expecting.