[Bug 2121215] Re: arm64 and arm64+largemem questing installs fail with VFS: Unable to mount root fs on unknown-block(0, 0)

[Dan Bungert]

** Changed in: livecd-rootfs (Ubuntu)
    Milestone: None => ubuntu-25.10

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Title:
  arm64 and arm64+largemem questing installs fail with VFS: Unable to
  mount root fs on unknown-block(0,0)

Status in livecd-rootfs package in Ubuntu:
  New

Bug description:
  Logs attached. For over a week now, the arm64 and arm64+largemem ISOs
  fail to install, and aren't getting promoted. I'm not entirely sure of
  the root cause, I've struggled to dig further. Here's the VM xml:

  ```
  <domain type="kvm">
    <name>utah-6281-questing-live-server-arm64+largemem</name>
    <uuid>7de8d542-662d-46bb-aa9e-bee16ed84e8a</uuid>
    <memory>1048576</memory>
    <currentMemory>1048576</currentMemory>
    <vcpu>1</vcpu>
    <cpu mode="host-passthrough" />
    <os>
      <type arch="aarch64" machine="virt">hvm</type>
      <loader readonly="yes" type="pflash">/usr/share/AAVMF/AAVMF_CODE.fd</loader>
      <nvram template="/usr/share/AAVMF/AAVMF_VARS.fd">/var/lib/utah/vm/utah-6281-questing-live-server-arm64+largemem/nvram.fd</nvram>
      <boot dev="cdrom" />
    </os>
    <features>
      <acpi />
    </features>
    <clock offset="utc" />
    <on_poweroff>destroy</on_poweroff>
    <on_reboot>destroy</on_reboot>
    <on_crash>destroy</on_crash>
    <devices>
      <controller type="scsi" index="0" model="virtio-scsi" />
      <disk type="file" device="cdrom">
        <driver name="qemu" type="raw" />
        <target dev="sdc" bus="scsi" />
        <readonly />
      <source file="/var/lib/utah/vm/utah-6281-questing-live-server-arm64+largemem/tmp14yaow03/questing-live-server-arm64+largemem.iso" /></disk>
      <interface type="network">
        <mac address="52:54:00:2b:6e:ca" />
        <source network="default" />
        <model type="virtio" />
      </interface>
    <disk type="file" device="disk"><driver name="qemu" type="qcow2" cache="unsafe" /><serial>disk-0</serial><source file="/var/lib/utah/vm/utah-6281-questing-live-server-arm64+largemem/disk0.qcow2" /><target dev="vda" bus="virtio" /></disk><disk type="file" device="disk"><driver name="qemu" type="raw" cache="unsafe" /><serial>disk-1</serial><source file="/var/lib/utah/vm/utah-6281-questing-live-server-arm64+largemem/tmp14yaow03/answers.img" /><target dev="vdb" bus="virtio" /></disk><serial type="file"><source path="/jenkins/servers/platformqa/workspace/ubuntu-questing-live-server-arm64+largemem-smoke-default/ARCHVARIANT/default/NODE/iso-testing-arm64+largemem/log/utah-6281.syslog.log" /><target port="0" /></serial></devices>
  </domain>
  ```

  And the relevant dmesg snippet:

  ```
  [    1.671218] clk: Disabling unused clocks
  [    1.672280] PM: genpd: Disabling unused power domains
  [    1.673612] md: Waiting for all devices to be available before autodetect
  [    1.673701] md: If you don't use raid, use raid=noautodetect
  [    1.676462] md: Autodetecting RAID arrays.
  [    1.677474] md: autorun ...
  [    1.677540] md: ... autorun DONE.
  [    1.679015] Failed to create /dev/root: -28
  [    1.680198] /dev/root: Can't lookup blockdev
  [    1.681294] VFS: Cannot open root device "" or unknown-block(0,0): error -2
  [    1.681364] Please append a correct "root=" boot option; here are the available partitions:
  [    1.681431] fd00        16777216 vda 
  [    1.681506]  driver: virtio_blk
  [    1.681565] fd10            1024 vdb 
  [    1.681614]  driver: virtio_blk
  [    1.681670] 0b00         2214300 sr0 
  [    1.681717]  driver: sr
  [    1.681772] List of all bdev filesystems:
  [    1.681822]  ext3
  [    1.681868]  ext2
  [    1.681918]  ext4
  [    1.681968]  squashfs
  [    1.682017]  vfat
  [    1.682067]  fuseblk
  [    1.682171] 
  [    1.682230] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
  [    1.694047] CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.15.0-4-generic-64k #4-Ubuntu PREEMPT(voluntary) 
  [    1.695968] Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
  [    1.697279] Call trace:
  [    1.697762]  show_stack+0x38/0xa0 (C)
  [    1.698473]  dump_stack_lvl+0x84/0xc0
  [    1.699179]  dump_stack+0x1c/0x40
  [    1.699818]  panic+0x1a8/0x3e0
  [    1.700409]  mount_root_generic+0x204/0x2e8
  [    1.701214]  mount_root+0xa8/0xd8
  [    1.701861]  prepare_namespace+0x1e0/0x290
  [    1.702648]  kernel_init_freeable+0x1a4/0x1e0
  [    1.703481]  kernel_init+0x3c/0x190
  [    1.704158]  ret_from_fork+0x10/0x20
  [    1.704898] Kernel Offset: disabled
  [    1.705590] CPU features: 0x0000,00000268,01000400,0200021b
  [    1.706663] Memory Limit: none
  [    1.707258] ---[ end Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0) ]---
  ```

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