When I saw Google's proposed native-code execution plug-in earlier this week, my initial reaction was: "Don't we already have those, and they're called exploits?"
I decided to mull it over a bit, and I still don't like it. While the idea of sandboxed native x86 execution based on real-time analysis of instruction sequences makes for a great Ph.D. thesis or 20%-time project, it sounds like an awfully large attack surface for questionable benefit.
Here's what I'd like to know from a practical nuts and bolts point-of-view: how many "compute-intensive" scenarios could not be implemented effectively using either (1) the Java VM or (2) a plug-in based on widely-available open-source machine virtualization, running a tiny Linux snapshot.
While the JVM falls short of native code in some places, it can be as fast -- or even faster -- in other cases (faster because the runtime behavior provides opportunities for on-the-fly optimization beyond what is known at compile time). Yes, there are issue with clients not all having the latest Java version -- but that seems a small issue compared with the operational issue of deploying a brand-new plug-in or browser (Chrome).
Another approach is to use a plug-in that runs a virtual machine, which in turn runs a small Linux (started from a static snapshot). User-mode code should run at approximately native speed in the VM, which should cover the pure computation cases. In a rare situation where someone wants to run an OLTP file-based db (which would behave badly in a VM environment) or get access to hardware-accelerated OpenGL facililties, specific well-defined services could be created on the host, which code could access via driver "ports" installed in the VM image.
These approaches to the security boundary -- Java VM and x86 VM -- are well-known, long-tested and based essentially on a white-list approach to allowing access to the physical host. While Google's idea is intriguing, it sounds as though it's based on black-listing (albeit with dynamic discovery rather than a static list) of code. I'm not yet convinced it's necessary or safe.