Re: Xen is a feature

Avi Kivity
2009-06-07T12:48:37Z

Ingo Molnar wrote:
> * 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.
>

Yeah, I lost the context. I should reduce my own context switching.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.