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[solved] qemu (at x86 created): cannot mount rootfs at amd64
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toralf
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PostPosted: Thu Dec 18, 2014 10:12 am    Post subject: [solved] qemu (at x86 created): cannot mount rootfs at amd64 Reply with quote

I created a qcow2 image at a x86 system, imported it here at a 64 bit system.

It boots fine, but the kernel tells me:"Kernel panic, unable to mount rootfs on unknown block (0,0)"
The / of the image is btrfs, /boot ext2 IIRC, and btrfs is compiled in the host - b/c it is the default fs here. The ext234 are now kernel modules.

Any hints ?


Last edited by toralf on Sat Dec 20, 2014 6:09 pm; edited 1 time in total
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NeddySeagoon
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PostPosted: Thu Dec 18, 2014 6:47 pm    Post subject: Reply with quote

toralf,

Code:
unknown block (0,0)

Whatever your VM provides as a hard disk chipset, or the virtio driver is missing from your guest kernel.
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NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.
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toralf
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PostPosted: Sat Dec 20, 2014 5:04 pm    Post subject: Reply with quote

NeddySeagoon wrote:
the virtio driver is missing from your guest kernel.
hhm, I re-used the x86 config from the old system, adapted it ofc to the new hardware - which driver name (==kernel option) do you mean ?
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NeddySeagoon
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PostPosted: Sat Dec 20, 2014 5:42 pm    Post subject: Reply with quote

toralf,

There are two ways to present virtual hard drives to the guest in a KVM.
If you run lspci it the guest you may see an IDE interface, a SATA interface or both.
This is the host faking some hardware for the guest. The guest will have /dev/sda and friends, which is what you are used.to seeing on real hardware.

You may also enable the tmo parts of the virtio driver. One on the hast and one in the guest.
The guest hard drive is now /dev/vda.

You need to know what you have tet up, since the former needs the hardware drivers foc the fake hardware and the latter does not.
You also need the correct root= and fstab entries in the guest.

The guest needs
Code:
   │ CONFIG_VIRTIO_BLK:                                                               │ 
  │                                                                                  │ 
  │ This is the virtual block driver for virtio.  It can be used with                │ 
  │ lguest or QEMU based VMMs (like KVM or Xen).  Say Y or M.


On the bare metal I have
Code:
  │ │    <M> PCI driver for virtio devices                                         │ │ 
  │ │    <M> Virtio balloon driver                                                 │ │ 
  │ │    <M> Platform bus driver for memory mapped virtio devices   

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NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.
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toralf
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PostPosted: Sat Dec 20, 2014 6:11 pm    Post subject: Reply with quote

thx - you pointed me into the right direction

I had to change in the guest the grub2 command line from "root=/dev/vda4" to "root=/dev/sda4"
(and I compiled now virtio_blk as a kernel module too - /me wonders, if that kernel option should be moved to the other virtio things ....)

So, now I'm in the guest and can can adapt fstab, dmcrypt and so on :-)
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