Re: [RFC PATCH 00/35] Xen i386 paravirtualization support

[Anthony Liguori <aliguori@us.ibm.com>]

Chris Wright wrote:
> * Anthony Liguori (aliguori@us.ibm.com) wrote:
>   
>> Chris Wright wrote:
>>     
>>> Xen also provides support for running directly on native hardware.
>>>       
>> Can someone elaborate on this?  Does this mean a Xen guest can run on 
>> bare metal?
>>     
>
> Yes.  See the Xen code for running the kernel in ring0 with Xen
> (supervisor_mode_kenel).  The hypercall_page is conditionally filled
> with hypercall traps or direct calls basically.
>   

Cool!  I didn't realize the supervisor_mode_kernel code was in the Xen 
tree code already.

Regards,

Anthony Liguori

>> Is there code available to make this work (it doesn't seem contained in 
>> this patchset)?  Has any performance analysis been done?
>>     
>
> I don't have any numbers.
>
>   
>> The numbers that have been posted with the VMI patches suggest that some 
>> rather tricky stuff is required to achieve native performance when 
>> running a guest on bare metal.  If this is not the case, it would be 
>> very interesting to know because it seems to be the hairiest part of the 
>> VMI patches.
>>     
>
> It is a hairy part of VMI.  They've done a nice job of handling the
> native case, and have interseting plans for improving the non-native
> case (inline where possible).  One of the differences is things that
> don't actually require hypercalls are already inline w/ Xen.  So it's
> conceivable that the performance hit is smaller than what VMI found
> without carefully inlining native code.
>
>   
>> Otherwise, if we want to support Xen guests on bare metal, it seems we 
>> would have to change things in the subarch code a bit to do something 
>> similar to VMI.
>>     
>
> It's a different approach.
>
> thanks,
> -chris
>