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

[Daniel Berrange <1875139@bugs.launchpad.net>]

> This issue is introduced in QEMU 4.2.0 (4.1.0 is working fine)

That's not neccessarily the case - with QEMU 4.2.0, libvirt switched
over to using the new -blockdev command line syntax. When you were
testing with 4.1.0, it would have been using the legacy -drive syntax.
So the change in behaviour is more likely related to the usage of
-blockdev, than any bug introduced in QEMU.

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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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