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

["Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>]

As many of the patches that are being posted are requiring more
and more changes to xenlinux/ia64, I would like to express a
concern.  Xen/x86 has taken many liberties with changes (primarily
for dom0) that the Linux community has not even seen yet as
the posted Linux patches for Xen -- and not yet accepted -- are
the less-invasive changes for domU only.

Per the attached news article, the Linux community is starting
to express a preference for the less-invasive VMware approach
and I fear that since we (Xen/ia64) are now following more of the
Xen/x86 approach, we are going to need to reverse course at
some point and back out many of the changes we are making to
Linux because the community will never accept them.

All on this list know that I am a fan of "transparent
paravirtualization"
but, in all of the early Xenlinux/ia64 paravirtualization work, I
have also worked for "optimized paravirtualization", which minimizes
the number of changes making only those required to maximize
performance.

I would like to urge caution for all Xen/ia64 developers.  We
should NOT be making changes to Linux just to "increase sharing
with Xen/x86 code" especially using the argument that we will
"reduce maintenance efforts" as I predict, in the long run, we
may actually be increasing our maintenance efforts.

Just my opinion...

Dan


========

http://news.zdnet.co.uk/software/applications/0,39020384,39263865,00.htm

Linux brings VMware into the fold 


Stephen Shankland
CNET News.com
April 18, 2006, 11:35 BST
 

Tell us your opinion 
 
As Xen makes strides into the SuSE and Red Hat kernels, the central
Linux team is trying a more egalitarian approach 
  
Linux programmers are moving toward a change that would put
virtualisation software from VMware on a more even footing with open
source rival Xen.

Xen was expected to be built tightly into the Linux kernel at the heart
of the open source operating system. But Andrew Morton, a key deputy to
Linux leader Linus Torvalds, is advocating an interface in the Linux
kernel that would let it work with any virtualisation foundation.

Virtualization generally refers to software and hardware that let a
single computer run multiple simultaneous operating system instances. It
is useful for making servers more efficient and isolating desktop
applications into non-interfering partitions. Xen, Microsoft and VMware
all are working on software called a hypervisor that governs how those
virtual machines get access to the hardware resources.

Morton said he prefers a neutral interface that works with any
hypervisor, rather than the Xen-specific patch to Linux that had been
envisioned.

"For a long time, it was thought that we'd just merge the Xen patches
as-is and be happy. But then, Linux would only run on Xen," Morton said.
Instead, VMware programmers suggested a documented, stable interface
between the kernel and the hypervisor - and they're preparing one, he
said.

"From a high-level design perspective, I think that VMware's point is a
good one, and that a general kernel-to-virtual machine interface is a
better thing than a Xen-only one," Morton said.

XenSource and VMware both are fine with the change, but VMware gets a
place at the table it lacked before.

"Anything that levels the playing field for different people - that's
going to be good for everyone, but certainly good for us as well," said
Dan Chu, the senior director of developer products at EMC subsidiary
VMware.

The issue comes up with a new generation of "paravirtualisation"
technology that offers better performance than VMware's current
approach, but that requires changes to the operating system. If software
companies adopt the kernel interface, that means that they and their
customers won't have to worry about different versions of software for
real or virtual machines, or different hypervisors, said Jack Lo,
VMware's senior director of research and development.

Morton said the Xen programmers haven't been active in the interface
work. "This has been floating around for a year, and I've heard precious
little from the Xen team on the topic," Morton said.

But Xen has a similar approach to the VMware interface, called VMI, and
the two are converging, said Xen founder Ian Pratt. "About 75 or 80
percent of the code structures are identical. It's that common ground
that hopefully should make its way into Linus' and Andrew's kernels
shortly," he said. "The discussion on all of those has, I think, yet to
begin."

Part of the problem has stemmed from branding issues, Pratt said. "They
object to putting patches into Linux having the 'Xen' prefix on all the
function names. We're miffed about it being called VMI, since we did the
hard work of interfaces and defining the patches," he said.

VMware didn't mean for VMI to stand for VMware - it's an abbreviation
for the generic Virtual Machine Interface - but the company is happy to
adjust names as necessary, Lo said.

The Open Source Development Labs has taken an active role in trying to
clean up the situation, Pratt added. "OSDL has volunteered to set up
meetings to get this stuff discussed," he said, noting they would
possibly be set up through a virtualisation task force.

How long until Xen merges?
Morton said the Xen programmers have work to do before they can expect
their patches to be incorporated into Linux. That inclusion into the
mainline kernel makes programming and certification tasks much easier
for Linux sellers such as Red Hat and Novell's SuSE.

"I've seen little from the Xen team, period. I see that Xen has been
snuck into Red Hat and possibly SuSE kernels, but right now I'd say it's
a long way from making it into mainline," he said. "We really haven't
even started looking at the code and discussing it."

The merge work is being handled by programmers at Red Hat and Novell's
SuSE who have closer ties with the main kernel programmers, Pratt said,
adding that he's optimistic it will be uncontroversial and take place
soon.

"There's a big block of code everyone can agree on. I think that will go
in very soon," Pratt said. "By September, I would hope that everything
is in there."

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