Hardware Virtualization Slower Than Software?

Jim Buzbee writes "Those you keeping up with the latest virtualization techniques being offered by both Intel and AMD will be interested in a new white paper by VMWare that comes to the surprising conclusion that hardware-assisted x86 virtualization oftentimes fails to outperform software-assisted virtualization. My reading of the paper says that this counterintuitive result is often due to the fact that hardware-assisted virtualization relies on expensive traps to catch privileged instructions while software-assisted virtualization uses inexpensive software substitutions. One example given is compilation of a Linux kernel under a virtualized Linux OS. Native wall-clock time: 265 seconds. Software-assisted virtualization: 393 seconds. Hardware-assisted virtualization: 484 seconds. Ouch. It sounds to me like a hybrid approach may be the best answer to the virtualization problem. "

Comments

Not just the CPU

[kripkenstein]

What matters is that small changes in the hardware make it possible to stop having to depend on costly, proprietary, and complex software--like that sold by VMware.

I am 100% in favor of cheap and open solutions. But I don't agree that this will soon be the case for virtualization. VMWare and the few other major vendors do a lot more than software virtualization of a CPU (which is all TFA was talking about). To have a complete virtualization solution, you need to also virtualize the rest of the hardware: storage, graphics, input/output, etc. In particular graphics is a serious issue (attaining hardware acceleration in a virtual environment safely), which from last I heard VMWare were working hard on.

Furthermore, Virtualization complements well with software that can migrate VMs (based on load or failure), and so forth. So, even if hardware CPU virtualization is to be desired - I agree with you on that - that won't suddenly make virtualization as a whole a simple task.

Re:The correct conclusion is more limited

[TheRaven64]

The easiest architecture to virtualise is the Alpha. It had a single privileged instruction, and all that did was shift to a higher privilege mode (which had a few shadow registers available) and then jump to an address in firmware. The firmware could be replaced by using one of these calls. If you wanted to virtualise it then you could do so trivially be replacing the firmware with something that would check or permute the arguments and then vector off into the original firmware.

It also had a few other advantages. Since you were adding virtual instructions, they all completed atomically (you can't pre-empt a process in the middle of an instruction). This meant you could put things like thread locking instructions in the PALCode and not require any intervention from the OS to run them. The VMS PALCode, for example, had a series of instructions for appending numbers to queues. These could be used to implement very fast message passing between threads (process some data, store it somewhere, then atomically write the address to the end of a queue) with no need to perform a system call (which meant no saving and loading of the CPU state, just jumping cheaply into a mode that could access a few more registers).