Re: Xen is a feature

Ingo Molnar
2009-06-07T10:37:08Z

* Avi Kivity <avi@redhat.com> wrote:

> Ingo Molnar wrote:
>>> There is in fact a way to get dom0 support with nearly no changes to
>>> Linux, but it involves massive changes to Xen itself and requires
>>> hardware support: run dom0 as a fully virtualized guest, and assign
>>> it all the resources dom0 can access. It's probably a massive effort
>>> though.
>>>
>>> I've considered it for kvm when faced with the "I want a thin
>>> hypervisor" question: compile the hypervisor kernel with PCI support
>>> but nothing else (no CONFIG_BLOCK or CONFIG_NET, no device drivers),
>>> load userspace from initramfs, and assign host devices to one or more
>>> privileged guests. You could probably run the host with a heavily
>>> stripped configuration, and enjoy the slimness while every interrupt
>>> invokes the scheduler, a context switch, and maybe an IPI for good
>>> measure.
>>>
>>
>> This would be an acceptable model i suspect, if someone wants a 'slim
>> hypervisor'.
>>
>> We can context switch way faster than we handle IRQs. Plus in a
>> slimmed-down config we could intentionally slim down aspects of the
>> scheduler as well, if it ever became a measurable performance issue.
>> The hypervisor would run a minimal user-space and most of the
>> context-switching overhead relates to having a full-fledged user-space
>> with rich requirements. So there's no real conceptual friction between
>> a 'lean and mean' hypervisor and a full-featured native kernel.
>>
>
> The context switch would be taken by the Xen scheduler, not the Linux
> scheduler. [...]

The 'slim hypervisor' model i was suggesting was a slimmed down
_Linux_ kernel.

Ingo