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[Xen-devel] RE: [Xen-ia64-devel] Urge caution on Xenlinux code changes

To: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
Subject: [Xen-devel] RE: [Xen-ia64-devel] Urge caution on Xenlinux code changes
From: Alex Williamson <alex.williamson@xxxxxx>
Date: Thu, 20 Apr 2006 13:48:04 -0600
Cc: "Magenheimer, Dan \(HP Labs Fort Collins\)" <dan.magenheimer@xxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, xen-ia64-devel@xxxxxxxxxxxxxxxxxxx
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On Thu, 2006-04-20 at 22:38 +0800, Tian, Kevin wrote:

> Then we SHOULD make changes to Linux to "increase sharing with 
> Xen/x86 code" especially using the argument that we will "reduce 
> maintenance efforts" AT THE SAME TIME:
>       - to support more missing features claimed in xen user manual, or
>       - to increase performance more efficiently
> And then finally improve xen/ia64 completeness.

   Some degree of code sharing makes sense and certainly expedites
development.  For instance, IOAPICs are probably similar enough to
IOSAPICs that it makes sense to implement paravirtualized interrupts in
a very similar way between Xen/x86 and Xen/ia64.  I wouldn't consider
this so much "sharing code" as implementing to the Xen hypervisor
interface.  When doing this, we need to make trade-offs of how closely
to follow Xen/x86 versus how would it make sense to expand the interface
to better meet the needs of both architectures.

   It seems to me that the best way to be prepared for whatever happens
upstream is to move forward and ensure that Xen/ia64 has the same
functionality as Xen/x86 using a set of virtual machine interfaces
common between the two.  This would not only add credibility to the Xen
virtual machine interface, but it will also ensure that the interfaces
we need exist in some form when we try to sort out what eventually goes
into upstream.

> To me, most efforts currently on xen/ia64 community are actually 
> following above direction.
> So my point is simple: let's just do things right...

   I agree, I think we're mainly on the right track.  We're still behind
in functionality, and it's hard to be an equal partner with Xen/x86 when
we have so many features yet to be implemented.  However, as Dan points,
we do need to be cautious when modifying Linux/ia64 for Xen/ia64.  We
need to try to make changes that will be acceptable to upstream
Linux/ia64 and realize if we're trying to fit a square peg into a round
hole ;^)  Thanks,


Alex Williamson                             HP Linux & Open Source Lab

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