[bonzini]

No, KVM is also a type 1 hypervisor but it doesn't attempt (with the exception of pKVM and of hardware protection features like SEV, neither of which is routinely used by cloud workloads) to protect the guest from a malicious host.

[Vogtinator]

KVM is a type 2 hypervisor as the "Dom 0" kernel has full HW access. Other guests are obviously isolated as configured and are like special processes to userspace.

It gets a bit blurry on AArch64 without and with VHE (Virtual Host Extensions) as without VHE (< ARMv8.1) the kernel runs in EL1 ("kernel mode") most of the time and escalates to EL2 ("hypervisor mode") only when needed, but with VHE it runs at EL2 all the time. (ref. https://lwn.net/Articles/650524/)

[bonzini]

No, "type 2" is defined by Goldberg's thesis as "The VMM runs on an extended host [53,75], under the host operating system", where:

* VMM is treated as synonymous with hypervisor

* "Extended host" is defined as "A pseudo-machine [99], also called an extended machine [53] or a user machine [75], is a composite machine produced through a combination of hardware and software, in which the machine's apparent architecture has been changed slightly to make the machine more convenient to use. Typically these architectural changes have taken the form of removing I/O channels and devices, and adding system calls to perform I/O and and other operations"

In other words, type 1 ("bare machine hypervisor") runs in supervisor mode and type 2 runs in user mode. QEMU running in dynamic binary translation mode is a type 2 VMM.

KVM runs on a bare machine, but it delegates some services to a less privileged component such as QEMU or crosvm or Firecracker. This is not a type 2 hypervisor, it is a type 1 hypervisor that follows security principles such as privilege separation.

[pjmlp]

Where in my comment did I refer explicitly to KVM feature set, or that it is used by cloud vendors?

[bonzini]

KVM is pretty much the only hypervisor that cloud vendors use these days.

So it's true that "most cloud workloads run on type 1 hypervisors" (KVM is one) but not that most cloud vendors/workloads run on microkernel-like hypervisors, with the exception of Azure.

[pjmlp]

You definitly didn't understood my commment.

[bonzini]

Then can you explain how cloud workloads is the revenge of the microkernel, since there is exactly 1 major cloud provider that uses a microkernel-like hypervisor?

[pjmlp]

By not running monolithic kernels on top of bare metal, rather virtualized, or even better with nested virtualization, thus throwing out the door all the supposedly performance advantages in the usual monolithic vs microkernel flamewar discussions, regarding context switching.

Additionally to make it to the next level, they run endless amount of container workloads.