Re: Xen is a feature

[Avi Kivity <avi@redhat.com>]

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.