HVM guest with file backed disks using loop devices ?

["Daniel P. Berrange" <berrange@redhat.com>]

I've been looking at an issue with HVM guests and file backed virtual disks.
A user reported that they were unable to create more than 8 HVM guests as
the hotplug scripts failed to find a free loop device on the 9th guest.

Ignoring the option of increasing max available loop devices in kernel, I'm
confused as to why  HVM guests need to allocate loopback devices at all.
The QEMU device model is perfectly happy access plain files directly - it
has no need for them to be bound to a loop device. Indeed if I look at an
active HVM guest, there is nothing using the loop device at all:

  # grep disk /etc/xen/demo 
  disk = [ "file:/xen/demo.img,hda,w", "file:/root/boot.iso,hdc:cdrom,r" ]
  # ps -axuwf | grep loop
  root     18631  0.0  0.0      0     0 ?        S<   13:32   0:00 [loop0]
  root     18673  0.0  0.0      0     0 ?        S<   13:32   0:00 [loop1]

  # lsof /dev/loop0
  # lsof /dev/loop1
  # lsof /root/boot.iso
  COMMAND   PID USER   FD   TYPE DEVICE    SIZE    NODE NAME
  qemu-dm 18551 root    6u   REG  253,0 6711296 1933665 /root/boot.iso
  # lsof /xen/demo.img
  COMMAND   PID USER   FD   TYPE DEVICE       SIZE    NODE NAME
  qemu-dm 18551 root    5u   REG  253,0 4294967297 1277954 /xen/demo.img

So qemu-dm has the two raw files open & nothing is using the loop devices. 


The loop devices are getting created for the HVM guest by the hotplug scripts,
in particular /etc/xen/scripts/block.  This script simply looks at the block
device type - either  phy:  or file:  and takes action accordingly - for the
latter it will always create a loop device. There does not appear to be any
logic in the hotplug script to check whether the guest is paravirt or HVM.

If I hack the 'block' hotplug script to rip out creation of the loopback device
for file:  disks, then HVM guests can still be created & appear to be able to
access their virtual disks without problems (obviously this isn't an actual
solution since it breaks paravirt file: disks).

I can think of two reasons for the creation of loop devices for HVM:

 - This was needed in the past, but is now obsolete & we simply forgot to
   turn off loop device code for HVM 
 - This is an accidental consequence changing the way HVM disks are
   configured (ie when we droppped  :ioemu tag from disks).

Can anyone who is more familiar with the history of HVM development shed
some light on this behaviour ?

I'd like to update the hotplug scripts to stop the loopback device being
created for HVM, but don't see any obvious data available in the hotplug
scripts which would allow me to distinguish between paravirt & HVM disk
configurations.

Regards,
Dan.
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