zlacker

>>hugodu+(OP)
Listen people, Firecracker is NOT A HYPERVISOR. A hypervisor runs right on the hardware. KVM is a hypervisor. Firecracker is a process that controls KVM. If you want to call firecracker (and QEMU, when used in conjunction with KVM) a VMM ("virtual machine monitor") I won't complain. But please please please, we need a word for what KVM and Xen are, and "hypervisor" is the best fit. Stop using that word for a user-level process like Firecracker.

>>gwd+j3
Nitpick: it’s not accurate to say that a hypervisor, by definition, runs right on the hardware. Xen (as a type-1 hypervisor) has this property; KVM (as a type-2 hypervisor) does not. It’s important to remember that the single core responsibility of a hypervisor is to divide hardware resources and time between VMs, and this decision-making doesn’t require bare-metal.

For those unfamiliar, the informal distinction between type-1 and type-2 is that type-1 hypervisors are in direct control of the allocation of all resources of the physical computer, while type-2 hypervisors operate as some combination of being “part of” / “running on” a host operating system, which owns and allocates the resources. KVM (for example) gives privileged directions to the Linux kernel and its virtualization kernel module for how to manage VMs, and the kernel then schedules and allocates the appropriate system resources. Yes, the type-2 hypervisor needs kernel-mode primitives for managing VMs, and the kernel runs right on the hardware, but those primitives aren’t making management decisions for the division of hardware resources and time between VMs. The type-2 hypervisor is making those decisions, and the hypervisor is scheduled by the OS like any other user-mode process.

>>foundr+Nq
Type-1 and type-2 hypervisor is terminology that should at this point be relegated to the past.

It was never popularly used in a way accurate to the origin of the classification - in the original paper by Popek and Goldberg talked about formal proofs for the two types and they really have very little to do with how the terms began being used in the 90s and 00s. Things have changed a lot with computers since the 70s when the paper was written and the terminology was coined.

So, language evolves, and Type-1 and Type-2 came to mean something else in common usage. And this might have made sense to differentiate something like esx from vmware workstation in their capabilities, but it's lost that utility in trying to differentiate Xen from KVM for the overwhelming majority of use cases.

Why would I say it's useless in trying to differentiate, say, Xen and KVM? Couple of reasons:

1) There's no performance benefit to type-1 - a lot of performance sits on the device emulation side, and both are going to default to qemu there. Other parts are based heavily on CPU extensions, and Xen and KVM have equal access there. Both can pass through hardware, support sr-iov, etc., as well.

2) There's no overhead benefit in Xen - you still need a dom0 VM, which is going to arguably be even more overhead than a stripped down KVM setup. There's been work on dom0less Xen, but it's frankly in a rough state and the related drawbacks make it challenging to use in a production environment.

Neither term provides any real advantage or benefit in reasoning between modern hypervisors.