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

Anthony Liguori
2006-03-22T17:51:01Z

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
>