KVM: Who’s your daddy?

I guess I never blogged about the VMWare solution to this. I wanted a node to detect if it was a VMWare guest, and if it was, to register the name of it’s host as an iClassify attribute. Originally the solution to know what VMWare server host a guest was on was to put it in it’s hostname when you created the node, like ‘vm03-somecrap’. That just sucked. With the information in iclassify, it was easy to write a ruby script to give me a list of hosts and their guests. And that worked alright. I’d run a script on the host against each guest something like:

#!/bin/bash
VMWARECMD=/usr/bin/vmware-cmd

# vmware config files returned by 'vmware-cmd -l' often have spaces, by default bash for treats spaces as a field seperator
IFS=$'\n'

if [ "$1" == "" ]; then
  echo $0: requires vmware host hostname as an argument
  exit 1
fi

for config in `$VMWARECMD -l` ; do
  $VMWARECMD "$config" setguestinfo host $1
done

Which one could then read on the guest if the vmware-guest tools where installed with:

/usr/sbin/vmware-guestd --cmd 'info-get guestinfo.host'

Finding the syntax for vmware-guestd took forever. I don’t know why I didn’t blog that.

I want to do something similar for KVM. I use libvirt, so the first step was looking at the configuration file options for libvirt and the command line options for qemu to see if anything nice matched up. I played a bit with the serial pipe option, which you can use even though it’s not listed on the libvirt page because I saw it in the source, but I couldn’t get it to do much. I had thought about having a daemon return hostname or such to the FIFO whenever queried with a \n or something.

I figured I could find something to pass to the kernel cmdline in an append option, but I really don’t want to have to template the libvirt template files just to stuff a hostname in there.

I really want something I can pass to SMBIOS that I can pull with dmidecode on the guest. I started digging into the LinuxBIOS/coreboot code that comes with the ubuntu kvm source package and in bios/rombios32.c, I found a function called uuid_probe that sets the uuid variable. I just posted to the coreboot mailing list asking about it. #qemu on irc.freenode.net just got me this when I asked about it:

15:29 < aliguori> in linux bios?  it's stuff that shouldn't be there

Googling for 0×564d5868 revealed it’s indicative of the VMWare backdoor stuff. This lead me to what appears to be some emulation of this called VMPort in QEMU. I have no idea yet what it does, if it ever worked, and thus if I can interact with it yet.