Re: VMWARE/ESXi Linux

In article <vin68p$3sjr$4@dont-email.me>,
Arne Vajhøj  <arne@vajhoej.dk> wrote:

On 11/28/2024 8:24 AM, Dan Cross wrote:

So Goldberg defined two "types" of hypervisor in his
dissertation: Types 1 and 2.  Of course, this is an over
simplification, and those of us who work on OSes and hypervisors
understand that these distinctions are blurry and more on a
continuum than hard and fast buckets, but to a first order
approximation these categories are useful.
 
Roughly, a Type-1 hypervisor is one that runs on the bare metal
and only supports guests; usually some special guest is
designated as a trusted "root VM".  Xen, ESXi, and Hyper-V are
examples of Type-1 hypervisors.
 
Again, roughly, a Type-2 hypervisor is one that runs in the
context of an existing operating system, using its services and
implementation for some of its functionality; examples include
KVM (they _say_ it's type 1, but that's really not true) and
PA1050.  Usually with a Type-2 HV you've got a userspace program
running under the host operating system that provides control
functionality, device models, and so on.  QEMU is an example of
such a thing (sometimes, confusingly, this is called the
hypervisor while the kernel-resident component, is called the
Virtual Machine Monitor, or VMM), but other examples exist:
CrosVM, for instance.

>
I think the relevant distinction is that type 1 runs in the
kernel while type 2 runs on the kernel.

No.  They both run in supervisor mode.  On x86, this is even
necessary; the instructions to enter guest mode are privileged.

Go back to Goldberg's dissertation; he discusses this at length.

KVM runs in Linux not on Linux. Which makes it type 1.

Nope.  KVM is dependent on Linux at this point.  The claim that
it is a type-1 hypervisor is predicated on the idea that it was
separable from Linux, but I don't think anyone believes that
anymore.

- Dan C.