[Bug 1875139] Re: Domain fails to start when 'readonly' device not writable

[A van Schie <1875139@bugs.launchpad.net>]

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 75, in cb_wrapper
    callback(asyncjob, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 111, in tmpcb
    callback(*args, **kwargs)
  File "/usr/share/virt-manager/virtManager/object/libvirtobject.py", line 66, in newfn
    ret = fn(self, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/object/domain.py", line 1279, in startup
    self._backend.create()
  File "/usr/lib/python3.7/site-packages/libvirt.py", line 1152, in create
    if ret == -1: raise libvirtError ('virDomainCreate() failed', dom=self)
libvirt.libvirtError: internal error: process exited while connecting to monitor: 2020-04-26T07:29:47.463835Z qemu-system-x86_64: -blockdev {"driver":"host_device","filename":"/var/lib/libvirt/vmportage/rootdisk","aio":"native","node-name":"libvirt-3-storage","cache":{"direct":true,"no-flush":false},"auto-read-only":true,"discard":"unmap"}: The device is not writable: Permission denied

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https://bugs.launchpad.net/bugs/1875139

Title:
  Domain fails to start when 'readonly' device not writable

Status in QEMU:
  New

Bug description:
  This issue is introduced in QEMU 4.2.0 (4.1.0 is working fine)

  My root disk is a LVM2 volume thin snapshot that is marked as read-only
  But when I try to start the domain (using virt-manager) I get the following error:

  Error starting domain: internal error: process exited while connecting
  to monitor: 2020-04-26T06:55:06.342700Z qemu-system-x86_64: -blockdev
  {"driver":"host_device","filename":"/dev/vg/vmroot-20200425","aio":"native
  ","node-name":"libvirt-3-storage","cache":{"direct":true,"no-
  flush":false},"auto-read-only":true,"discard":"unmap"} The device is
  not writable: Permission denied

  Changing the lvm snapshot to writeable allows me to start the domain.
  (Making it changes possible during domain is running)

  I don't think QEMU should fail when it can't open a (block) device when the read-only option is set.
  (why is write access needed?)

  Reproduce steps:
  * Create LVM read-only volume (I don't think any data is needed)
  * Create domain with read-only volume as block device
  * Try to start the domain

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