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

[Martin Ichilevici de Oliveira]

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

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
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

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'

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 ]---

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