Re: State of current pv_ops backend for KVM?

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Rusty Russell wrote:
> On Tue, 2007-05-29 at 17:00 +0300, Avi Kivity wrote:
>   
>> Anthony Liguori wrote:
>>     
>>> Avi Kivity wrote:
>>>       
>>>> I think we should aim for a full direct paging implementation, where 
>>>> the cr3 cache is just the cherry on top that makes context switches 
>>>> proceed at native speed.  Jun has a paper on it for OLS and I hope 
>>>> patches too.
>>>>         
>>> That's a bit more ambitious than I was thinking.  I would be very 
>>> happy with a pv timer, interrupt controller, and some basic CPU 
>>> optimizations.  I'm still not convinced the perf benefits of direct 
>>> paging justify the annoyances that come with it (guests being aware of 
>>> pfns).  
>>>       
>> I thought that with paravirt_ops, all the awareness is limited to the 
>> paravirt implementation, and nothing in the core kernel is actually 
>> aware of host pfns.
>>     
>
> It is, but now you've broken the abstraction you get other messes.  You
> can no longer simply swap out pages behind the guest's back, copy on
> write or combine identical pages or other tricks without synchronizing
> with the guest.
>
>   

These are good points.

> It can be done, 

Maybe we'll get something from the s390 port.

> but it's not clear to me that verifying PTEs is that
> much faster than shadowing them if you're prepared to be that invasive
> in the guest anyway.
>   
This can probably be measured on Xen which can switch from direct to
shadow mode on runtime.

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


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