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Re: [Xen-devel] [RFC] VMI for Xen?

Ian Pratt wrote:
Just some food for thought, I did a simple analysis of the size of Xen Linux patches verses the VMI patches. To make things more fair, I removed everything from the XenoLinux port accept for the i386 xen subarch (so no drivers and no support for other architectures).

Your analysis is still taking all the patches to make dom0 functionality
work, which accounts for a lot of changes and is totally outside of the
scope of VMI.

We've been working with a bunch of RH/Novell folks to create a stripped
down domU-only Xen patch that would be a fairer comparison.

We're currently looking though the new VMI patchset to see whether
there's anything worth having, in particular, if there's anything that
should be added to our patch to support what we can deduce that their
hypervisor probably needs. We've also talking to the VirtualIron team to
make sure we're inclusive as possible.

The current VMI patchset couldn't support Xen's direct-mode (non shadow)
MMU virtualization, and hasn't really been thought through properly for
SMP. I'm sure we'll get something worked out that keeps everyone happy.
Actually, the new spec does support non-shadow paging. All page table updates are done via the VMI ROM. Normally, these are just normal get/set operations but they could be get/sets with p2m looks. The first version of the spec definitely didn't support non-shadow paging though.

We'll see whether it's actually implementable when the wedge is released though. I wouldn't be surprised if they are using shadow paging.

The new spec does actually introduce SMP stuff but yeah, you're probably right, it may not have been thought through enough... The biggest gaps that I can find appear to be DMA support and event channels. I think to properly support event channel you need a bit more pervasive changes. I think DMA is a pretty simply change (you just have to update the DMA bus translation functions to do a p2m lookup).


Anthony Liguori

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