Re: [Qemu-discuss] Help with -kernel option (unable to mount root fs)

[Jakob Bohm]

On 24-09-2014 03:18, Martin Ichilevici de Oliveira wrote:
> Hello,
>
> I'm trying to use the -kernel option of QEMU, in order to later debug
> Linux with GDB, but I've been unable to boot the system.
>
> My setup: a working CentOS 7 with a manually compilled kernel (3.17-rc5).
> If I simply boot the image with qemu-system-x86_64, it works fine. So
> I copied the bzImage out of the VM and ran:
>
> $ qemu-system-x86_64 -m 4G -hda image.img -kernel bzImage -append "root=/dev/sda 
> console=ttyS0" -nographic
root=/dev/sda is clearly wrong, that is the whole unpartitioned disk,
not the root partition inside it.  You probably want /dev/sda1.  But
morebelow...
> The system failed to boot (log at the end of this email), but basically
> it's complaining about being unable to mount the root filesystem.
>
> I thought I'd used the wrong /dev/sdX on the append option, so I
> checked with
>
> # fdisk -l
Did you run this command on the guest?
> Disk /dev/sda: 32.2 GB, 32212254720 bytes, 62914560 sectors
> Units = sectors of 1 * 512 = 512 bytes
> Sector size (logical/physical): 512 bytes / 512 bytes
> I/O size (minimum/optimal): 512 bytes / 512 bytes
> Disk label type: dos
> Disk identifier: 0x000a2200
>
> Device Boot      Start         End      Blocks   Id  System
> /dev/sda1   *        2048     1026047      512000   83  Linux
> /dev/sda2         1026048    62914559    30944256   8e  Linux LVM
I take that to mean that /dev/sda1 is a small 512MB partition
containing grub, kernels, initrd images etc.  Probably mounted as
"/boot" by the guest once it is running.

/dev/sda2 is subpartitioned using LVM and I am not sure the built
in root mounting code in the kernel can handle that without help
from user mode tools in an initrd.

You need to look in the grub config for the file name of that initrd
image, then pass that to qemu along with the kernel image and the
required kernel commandline.  The kernel will run the initrd before
it tries to mount root, and the initrd (presumably generated
automatically by CentOS) will then load the needed LVM tools to
find /dev/mapper/centos-root and mount it as root.

> Disk /dev/mapper/centos-swap: 3221 MB, 3221225472 bytes, 6291456 sectors
> Units = sectors of 1 * 512 = 512 bytes
> Sector size (logical/physical): 512 bytes / 512 bytes
> I/O size (minimum/optimal): 512 bytes / 512 bytes
>
> Disk /dev/mapper/centos-root: 10.5 GB, 10485760000 bytes, 20480000 sectors
> Units = sectors of 1 * 512 = 512 bytes
> Sector size (logical/physical): 512 bytes / 512 bytes
> I/O size (minimum/optimal): 512 bytes / 512 bytes
>
> Disk /dev/mapper/centos-home: 18.0 GB, 17976786944 bytes, 35110912 sectors
> Units = sectors of 1 * 512 = 512 bytes
> Sector size (logical/physical): 512 bytes / 512 bytes
> I/O size (minimum/optimal): 512 bytes / 512 bytes
>
> Not many options here... I tried with /dev/sda1, /dev/sda2 and 
> /dev/mapper/centos-root,
> but all got the same error.
>
> Finally, I checked the grub entry for that kernel and it contains:
> set root='hd0,msdos1'
I guess this line is just there to help grub find the kernel and
initrd, then otherparameters will make the initrd tell the kernel
where the real root is.

Note that grub, for unfathomable reasons, has its own weird naming
of partitions. hd0,msdos1probably means /dev/sda1 (the small boot
partition), or it could mean /dev/sda2 (the LVMcontainer, which
obviously won't be used directly as root).
> But I'm not really sure how to use this information.
>
> Any help is appreciated.
>
> Thank you,
> Martin
>
> Kernel panic log:
> (...)
> [    2.278415] List of all partitions:
> [    2.279764] No filesystem could mount root, tried:
> [    2.281594] Kernel panic - not syncing: VFS: Unable to mount root fs on 
> unknown-block(0,0)
> [    2.282071] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.17.0-rc6+ #1
> [    2.282071] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 
> 1.7.5-20140617_173321-var-lib-archbuild-testing-x86_64-tobias 04/01/2014
> [    2.282071]  0000000000000000 00000000724949f3 ffff88003daabd60 
> ffffffff81686bcd
> [    2.282071]  ffffffff818ba3c0 ffff88003daabde8 ffffffff81683afb 
> ffffffff00000010
> [    2.282071]  ffff88003daabdf8 ffff88003daabd90 00000000724949f3 
> 00000000724949f3
> [    2.282071] Call Trace:
> [    2.282071]  [<ffffffff81686bcd>] dump_stack+0x45/0x56
> [    2.282071]  [<ffffffff81683afb>] panic+0xd5/0x209
> [    2.282071]  [<ffffffff81b3d5ec>] mount_block_root+0x2a4/0x2b3
> [    2.282071]  [<ffffffff81b3d64e>] mount_root+0x53/0x56
> [    2.282071]  [<ffffffff81b3d78d>] prepare_namespace+0x13c/0x174
> [    2.282071]  [<ffffffff81b3d25a>] kernel_init_freeable+0x23d/0x261
> [    2.282071]  [<ffffffff8167a3d0>] ? rest_init+0x80/0x80
> [    2.282071]  [<ffffffff8167a3de>] kernel_init+0xe/0xf0
> [    2.282071]  [<ffffffff8168ebbc>] ret_from_fork+0x7c/0xb0
> [    2.282071]  [<ffffffff8167a3d0>] ? rest_init+0x80/0x80
> [    2.282071] Kernel Offset: 0x0 from 0xffffffff81000000 (relocation range: 
> 0xffffffff80000000-0xffffffff9fffffff)
> [    2.282071] ---[ end Kernel panic - not syncing: VFS: Unable to mount root 
> fs on unknown-block(0,0)
> [    2.282071] general protection fault: fff2 [#1] SMP
> [    2.282071] Modules linked in:
> [    2.282071] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.17.0-rc6+ #1
> [    2.282071] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 
> 1.7.5-20140617_173321-var-lib-archbuild-testing-x86_64-tobias 04/01/2014
> [    2.282071] task: ffff88003daa0000 ti: ffff88003daa8000 task.ti: 
> ffff88003daa8000
> [    2.282071] RIP: 0010:[<ffffffff81683beb>]  [<ffffffff81683beb>] 
> panic+0x1c5/0x209
> [    2.282071] RSP: 0000:ffff88003daabd70  EFLAGS: 00000282
> [    2.282071] RAX: 0000000000000057 RBX: ffffffff818ba3c0 RCX: 0000000000000057
> [    2.282071] RDX: 0000000000000000 RSI: 0000000000000046 RDI: 0000000000000046
> [    2.282071] RBP: ffff88003daabde8 R08: 00000000000067b8 R09: 0000000000000096
> [    2.282071] R10: ffffffff81b177c0 R11: 000000000000018b R12: ffff88003daabdf8
> [    2.282071] R13: 0000000000000000 R14: 0000000000000000 R15: ffff88003d710000
> [    2.282071] FS:  0000000000000000(0000) GS:ffff88003fc00000(0000) 
> knlGS:0000000000000000
> [    2.282071] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> [    2.282071] CR2: 0000000000000000 CR3: 00000000019c4000 CR4: 00000000000006f0
> [    2.282071] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [    2.282071] DR3: 0000000000000000 DR6: 0000000000000000 DR7: 0000000000000000
> [    2.282071] Stack:
> [    2.282071]  ffffffff00000010 ffff88003daabdf8 ffff88003daabd90 
> 00000000724949f3
> [    2.282071]  00000000724949f3 ffff88003daabe08 0000000000000012 
> 0000000000000000
> [    2.282071]  000000000000000a 000000000000fffb ffff88003d710000 
> 0000000000000000
> [    2.282071] Call Trace:
> [    2.282071]  [<ffffffff81b3d5ec>] mount_block_root+0x2a4/0x2b3
> [    2.282071]  [<ffffffff81b3d64e>] mount_root+0x53/0x56
> [    2.282071]  [<ffffffff81b3d78d>] prepare_namespace+0x13c/0x174
> [    2.282071]  [<ffffffff81b3d25a>] kernel_init_freeable+0x23d/0x261
> [    2.282071]  [<ffffffff8167a3d0>] ? rest_init+0x80/0x80
> [    2.282071]  [<ffffffff8167a3de>] kernel_init+0xe/0xf0
> [    2.282071]  [<ffffffff8168ebbc>] ret_from_fork+0x7c/0xb0
> [    2.282071]  [<ffffffff8167a3d0>] ? rest_init+0x80/0x80
> [    2.282071] Code: c3 64 eb b1 83 3d 15 35 70 00 00 74 05 e8 1e 32 a1 ff 48 c7 c6 
> 40 71 d8 81 48 c7 c7 60 be 8c 81 31 c0 e8 64 02 00 00 fb 66 66 90 <66> 66 90 45 
> 31 e4 e8 3a 49 aa ff 4d 39 ec 7c 18 41 83 f6 01 44
> [    2.282071] RIP  [<ffffffff81683beb>] panic+0x1c5/0x209
> [    2.282071]  RSP <ffff88003daabd70>
> [    2.282071] ---[ end trace 5c54a652a799b4be ]---


Enjoy

Jakob
--
Jakob Bohm, CIO, Partner, WiseMo A/S.  http://www.wisemo.com
Transformervej 29, 2730 Herlev, Denmark.  Direct +45 31 13 16 10
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