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SubjectRe: [RFC PATCH 00/35] Xen i386 paravirtualization support
* 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.

> 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
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