[Xen-devel] Installing from distribution CDs

[Anthony Liguori <aliguori@xxxxxxxxxx>]

Hi,

I recently tried to see what it would take in Xen to install a
distribution from a CD in domain > 0.  Here's my findings along with a
couple patches I used to find this.  These patches are proof of concepts
and quite ugly.

1) initrd support

Xen currently implements the ramdisk option which seems to simply pull
in a gzip'd file to a ramdisk and set that as the root.  Looking at the
initrd code in the kernel and reading through the man page, initrd is a
bit more complicated than that.

Among other things, the initrd process will check for a /linuxrc file
and execute it.  After execution, it will remount a new root and then
fall through to the normal init process.

SLES-9 at least uses linuxrc.  I imagine most distros do since it was
added specifically to help out distros.

2) CDROM support.  SLES-9 at least expects to have been booted from a
CDROM and looks for a /proc/sys/ entry to find out where that CDROM is. 
Right now Xen doesn't really support CDROMs.

The attached patches add /linuxrc to the init list (this will at least
let you get a bit into the installation process), add an old hack from
2.4 Xen to enable at least some response to MULTISESSION ioctls
(otherwise isofs won't mount the device), and also add a CDROM entry to
/proc/sys for hdc1.

A couple questions did come up though:

1) why isn't xen using the Linux initrd code?  The man page says that
initrd has unspecified features.  My fear is that some distros rely on
these features so reusing the initrd code would probably be best.

2) Have there been thoughts on how removable devices like CDROMs are
going to be supported?

BTW, you have to increase the ramdisk size to 64MB and add support for
minix to the kernel in order to boot off of  SLES cd.

Regards,

-- 
Anthony Liguori
Samba, Linux/Windows Interoperability
Linux Technology Center (LTC) - IBM Austin
E-mail: aliguori@xxxxxxxxxx
Phone: (512) 838-1208
Tie Line: 678-1208

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