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xen-devel
Re: [Xen-devel] [RFC] VMI for Xen?
Christian pointed out that the current Xen tree copies a number of files
and makes changes to them (renaming them to -xen). It's unclear how to
properly count this in my mind since if they went into the tree as-is
today, it would be a pure addition. Presumably, they would go in with
some conditional logic which would reduce the total insertion count
(dramatically) but also increase the diff size.
This probably helps explains why Chris is seeing such a dramatic
reduction too. It's becoming clear to me that when reworked, the Xen
changes are pretty much identical in code-size to VMI (give or take a
couple thousand slocs).
Regards,
Anthony Liguori
Anthony Liguori 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).
VMI:
124 files changed, 4964 insertions(+), 623 deletions(-)
Xen:
185 files changed, 31586 insertions(+), 142 deletions(-)
So the Xen port adds 6 times more code than VMI. I certainly don't
think VMI for Xen is going to be as fast as native Xen but I don't
know of anything that would cause a substantial change in VMI to add
the necessary optimizations that would result in a massive change in
the size of the patches.
I should also mention that one should also consider the size of the
VMI Xen ROM too. For the L4-based ROM that's going to be ~10k lines
but I expect that if the Xen hypercalls were adjusted a little bit, it
would drop much less. For instance, Xen already does platform device
emulation in the hypervisor for HVM domains so if that were reused it
would knock out a fair amount of the ROM code.
It seems like there are some merits to the VMI approach. Is there
something I'm missing? I admit I don't understand the XenoLinux
changes well enough to know with certainity if there's something major
that justifies the difference in size. I'm hoping someone can hit me
with a clue stick though if there is :-)
Regards,
Anthony Liguori
Anthony Liguori wrote:
I'm sure everyone has seen the drop of VMI patches for Linux at this
point, but just in case, the link is included below.
I've read this version of the VMI spec and have made my way through
most of the patches. While I wasn't really that impressed with the
first spec wrt Xen, the second version seems to be much more
palatable. Specifically, the code inlining and afterburner-style
padding seems like a really promising approach to native-speed single
kernel images. Also, this version seems much more friendly to p2m.
There are still a few things missing (like guest DMA support) but I
think the basic ideas are pretty sane. So what does everyone else
think? Is there anything within VMI that would inhibit some of Xen's
optimizations? Are there any disadvantages to a VMI-style approach
to the subarch changes?
How close are we to being able to merge our stuff with mainline?
Have we gotten feedback yet on how hard this is going to be? Would
VMI be an easier approach to inclusion in mainline?
Just thought it would be prudent to start a discussion here, at
least, about it...
http://lkml.org/lkml/2006/3/13/140
Regards,
Anthony Liguori
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