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

[A van Schie]

The domain.xml:
<domain type='kvm'>
  <name>rotest</name>
  <uuid>b4aa0288-8886-42df-abfd-4c8f729e1330</uuid>
  <memory unit='KiB'>2048000</memory>
  <currentMemory unit='KiB'>2048000</currentMemory>
  <vcpu placement='static'>2</vcpu>
  <os>
    <type arch='x86_64' machine='pc-i440fx-2.7'>hvm</type>
    <kernel>/var/lib/libvirt/pink/kernel</kernel>
    <cmdline>root=/dev/sda ro panic=300 systemd.show_status=1 
systemd.unit=graphical.target quiet</cmdline>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
  </features>
  <cpu mode='custom' match='exact' check='none'>
    <model fallback='forbid'>qemu64</model>
  </cpu>
  <clock offset='utc'>
    <timer name='rtc' tickpolicy='catchup'/>
    <timer name='pit' tickpolicy='delay'/>
    <timer name='hpet' present='no'/>
  </clock>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/bin/qemu-system-x86_64</emulator>
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw'/>
      <source dev='/dev/nvmvg/rotest'/>
      <target dev='sda' bus='scsi'/>
      <readonly/>
      <shareable/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <source dev='/dev/nvmvg/rotest-var'/>
      <target dev='sdb' bus='scsi'/>
      <address type='drive' controller='0' bus='0' target='0' unit='1'/>
    </disk>
    <controller type='pci' index='0' model='pci-root'/>
    <controller type='scsi' index='0' model='virtio-scsi'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' 
function='0x0'/>
    </controller>
    <controller type='usb' index='0' model='piix3-uhci'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' 
function='0x2'/>
    </controller>
    <input type='mouse' bus='ps2'/>
    <input type='keyboard' bus='ps2'/>
    <graphics type='spice' autoport='yes'>
      <listen type='address'/>
      <gl enable='no' rendernode='/dev/dri/by-path/pci-0000:00:02.0-render'/>
    </graphics>
    <video>
      <model type='virtio' heads='1' primary='yes'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' 
function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' 
function='0x0'/>
    </memballoon>
  </devices>
</domain>

--------------------------------------------------------------------------------

The qemu command:
2020-04-27 11:57:11.720+0000: starting up libvirt version: 6.0.0, qemu version: 
4.2.0, kernel: 5.4.28-gentoo, hostname: gentoo
LC_ALL=C \
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin \
HOME=/var/lib/libvirt/qemu/domain-10-rotest \
XDG_DATA_HOME=/var/lib/libvirt/qemu/domain-10-rotest/.local/share \
XDG_CACHE_HOME=/var/lib/libvirt/qemu/domain-10-rotest/.cache \
XDG_CONFIG_HOME=/var/lib/libvirt/qemu/domain-10-rotest/.config \
QEMU_AUDIO_DRV=spice \
/usr/bin/qemu-system-x86_64 \
-name guest=rotest,debug-threads=on \
-S \
-object 
secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-10-rotest/master-key.aes
 \
-machine pc-i440fx-2.7,accel=kvm,usb=off,dump-guest-core=off \
-cpu qemu64 \
-m 2000 \
-overcommit mem-lock=off \
-smp 2,sockets=2,cores=1,threads=1 \
-uuid b4aa0288-8886-42df-abfd-4c8f729e1330 \
-no-user-config \
-nodefaults \
-chardev socket,id=charmonitor,fd=32,server,nowait \
-mon chardev=charmonitor,id=monitor,mode=control \
-rtc base=utc,driftfix=slew \
-global kvm-pit.lost_tick_policy=delay \
-no-hpet \
-no-shutdown \
-boot strict=on \
-kernel /var/lib/libvirt/pink/kernel \
-append 'root=/dev/sda ro panic=300 systemd.show_status=1 
systemd.unit=graphical.target quiet' \
-device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 \
-device virtio-scsi-pci,id=scsi0,bus=pci.0,addr=0x5 \
-blockdev 
'{"driver":"host_device","filename":"/dev/nvmvg/rotest","node-name":"libvirt-2-storage","auto-read-only":true,"discard":"unmap"}'
 \
-blockdev 
'{"node-name":"libvirt-2-format","read-only":true,"driver":"raw","file":"libvirt-2-storage"}'
 \
-device 
scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,lun=0,device_id=drive-scsi0-0-0-0,share-rw=on,drive=libvirt-2-format,id=scsi0-0-0-0,bootindex=1
 \
-blockdev 
'{"driver":"host_device","filename":"/dev/nvmvg/rotest-var","node-name":"libvirt-1-storage","cache":{"direct":true,"no-flush":false},"auto-read-only":true,"discard":"unmap"}'
 \
-blockdev 
'{"node-name":"libvirt-1-format","read-only":false,"cache":{"direct":true,"no-flush":false},"driver":"raw","file":"libvirt-1-storage"}'
 \
-device 
scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,lun=1,device_id=drive-scsi0-0-0-1,drive=libvirt-1-format,id=scsi0-0-0-1,write-cache=on
 \
-spice port=5900,addr=192.168.1.9,disable-ticketing,seamless-migration=on \
-device virtio-vga,id=video0,max_outputs=1,bus=pci.0,addr=0x2 \
-device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x7 \
-sandbox 
on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny \
-msg timestamp=on
2020-04-27T11:57:11.804028Z qemu-system-x86_64: -blockdev 
{"driver":"host_device","filename":"/dev/nvmvg/rotest","node-name":"libvirt-2-storage","auto-read-only":true,"discard":"unmap"}:
 The device is not writable: Permission denied
2020-04-27 11:57:11.805+0000: shutting down, reason=failed

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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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