From: cross@spitfire.i.gajendra.net
In article ,
Arne Vajhøj 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.
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