Xen devel - Jan 2005 - Code size vs. linux source code compatibility in the hypervisor

Hello there!

I recently spend some time trying to better understand the hypervisor part of 
xen and decided to dive into the code in the xen subdirectory. David A. 
Wheeler''s ''SLOCCount'' reports about 53k lines of C
code there.

My first impression was that much of the code is in fact unused by Xen and 
mostly there, because it was salvaged from the Linux kernel. To aid my 
understanding of Xen I started to trim down on macros and functions. I was 
surprised to be able to actually remove about 8k lines of code and 1.5k macro 
definitions from the ACPI part! Xen still builds and runs for me:-) Since the 
compiler will optimize out unused code this reduction is not reflected in the 
compiled size of the hypervisor (mine is identical in size to the one build 
from yesterdays unstable tarball).

My question is: What is the stance of the core team on the issue of code size 
vs. linux source code compatibility? The less code the easier it is for 
others to get into the project, while keeping the complete linux code around 
might make moving new code over to xen easier. Are you interested in patches 
to remove unused code? Mine currently removes almost all the header files in 
include/acpi...

What do you think about #if 0 in code? I personally hate that: There is 
version tracking, so there is no reason to keep old code visible that way. 
What about all those "#ifdef CONFIG_FOO" lines from the linux kernel? 
Currently xen is rather fixed in the configuration it supports and uses. Is 
that a feature of xen (then those #ifdefs can go IMHO) or supposed to change 
over time?

As you might have noticed I have not used C too much for the last couple of 
years and never was a fan of cpp in the first place:-) This code rewriting 
all over the place keeps knocking me out of the flow of understanding what is 
happening.

-- 
Gruss,
Tobias

------------------------------------------------------------
Tobias Hunger           The box said: ''Windows 95 or better''
tobias@aquazul.com                     So I installed Linux.
------------------------------------------------------------

As part of the next release we plan to move most of the platform init
code into dom0. This includes PCI & IO-APIC setup etc for which we
currently need the ACPI parts. So hopefully most of the unnecessary code
you identified and probably more will disappear.
rolf
> -----Original Message-----
> From: xen-devel-admin@lists.sourceforge.net [mailto:xen-devel-
> admin@lists.sourceforge.net] On Behalf Of Tobias Hunger
> Sent: 29 January 2005 11:32
> To: xen-devel@lists.sourceforge.net
> Subject: [Xen-devel] Code size vs. linux source code compatibility in
the
> hypervisor
> 
> Hello there!
> 
> I recently spend some time trying to better understand the hypervisor
part
> of
> xen and decided to dive into the code in the xen subdirectory. David
A.
> Wheeler''s ''SLOCCount'' reports about 53k lines of
C code there.
> 
> My first impression was that much of the code is in fact unused by Xen
and
> mostly there, because it was salvaged from the Linux kernel. To aid my
> understanding of Xen I started to trim down on macros and functions. I
was
> surprised to be able to actually remove about 8k lines of code and
1.5k
> macro
> definitions from the ACPI part! Xen still builds and runs for me:-)
Since
> the
> compiler will optimize out unused code this reduction is not reflected
in
> the
> compiled size of the hypervisor (mine is identical in size to the one
> build
> from yesterdays unstable tarball).
> 
> My question is: What is the stance of the core team on the issue of
code
> size
> vs. linux source code compatibility? The less code the easier it is
for
> others to get into the project, while keeping the complete linux code
> around
> might make moving new code over to xen easier. Are you interested in
> patches
> to remove unused code? Mine currently removes almost all the header
files
> in
> include/acpi...
> 
> What do you think about #if 0 in code? I personally hate that: There
is
> version tracking, so there is no reason to keep old code visible that
way.
> What about all those "#ifdef CONFIG_FOO" lines from the linux
kernel?
> Currently xen is rather fixed in the configuration it supports and
uses.
> Is
> that a feature of xen (then those #ifdefs can go IMHO) or supposed to
> change
> over time?
> 
> As you might have noticed I have not used C too much for the last
couple
> of
> years and never was a fan of cpp in the first place:-) This code
rewriting
> all over the place keeps knocking me out of the flow of understanding
what
> is
> happening.
> 
> --
> Gruss,
> Tobias
> 
> ------------------------------------------------------------
> Tobias Hunger           The box said: ''Windows 95 or
better''
> tobias@aquazul.com                     So I installed Linux.
> ------------------------------------------------------------

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On Saturday 29 January 2005 13:24, Neugebauer, Rolf
wrote:
> As part of the next release we plan to move most of the platform init
> code into dom0. This includes PCI & IO-APIC setup etc for which we
> currently need the ACPI parts. So hopefully most of the unnecessary code
> you identified and probably more will disappear.
> rolf
Great!

Is there any paper available about the distribution of responsibilities 
between xen, dom0 and other domains in the next release? I saw several hints 
at that here on the list, but I''d like to get the big picture. I am
really
curious what you guys here came up with.

Is there a list of tasks and or bugs that can help newbies to get into xen? 
I''d love to contribute to your project, but so far I have not really
found a
good starting place.

-- 
Gruss,
Tobias

------------------------------------------------------------
Tobias Hunger           The box said: ''Windows 95 or better''
tobias@aquazul.com                     So I installed Linux.
------------------------------------------------------------