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
KVM is a type-1 hypervisor [1]

[1]: https://www.redhat.com/en/topics/virtualization/what-is-KVM

>>veber-+Bw
Not really, calling KVM a type-1 is a misunderstanding of what the “bare-metal” distinction is referring to. The real difference between the two types is whether the hypervisor owns the hardware or not. In the case of a type-1, the hypervisor runs below the kernel and controls access to the hardware, even for the kernel. In type-2, the hypervisor runs on the kernel, which owns the hardware, and must go through the kernel to use hardware resources.

>>compsc+yZ
But that's not how that works. KVM is as "bare-metal" in access to the system as ESXi is. The hypervisor code runs in ring 0 in both cases.