VFS: Cannot open root device "(null)" or unknown-block(0, 0): error -2

VFS: Cannot open root device "(null)" or unknown-block(0, 0): error -2

[jim.cromie]

Im trying to configure a minimal kernel,
that will work on a virtme / qemu / kvm stack

I did
make allnoconfig
virtme-configkernel --update

then I added BTRFS_FS, since thats the host fs.
of course it didnt work.

then tried DM, needed MD, added both,
again didnt help.

so I need to stop just guessing.
(admittedly, asking is worse ?)

heres how it actually goes:

9pnet: Installing 9P2000 support
sched_clock: Marking stable (3466602654, 29303733)->(4050551166, -554644779)
Btrfs loaded, crc32c=crc32c-generic, zoned=no
9pnet_virtio: no channels available for device /dev/root
VFS: Cannot open root device "(null)" or unknown-block(0,0): error -2
Please append a correct "root=" boot option; here are the available partitions:
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
CPU: 0 PID: 1 Comm: swapper Tainted: G        W         5.14.0-rc4+ #5
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.14.0-4.fc34 04/01/2014
Call Trace:
 dump_stack+0x1b/0x1d
 panic+0x95/0x202
 mount_block_root+0x204/0x268
 mount_root+0x5d/0x62


Heres a working  kernel (if overlarge config)

It has btrfs, md, which is why I took those guesses above

zswap: loaded using pool lzo/zbud
Key type ._fscrypt registered
Key type .fscrypt registered
Key type fscrypt-provisioning registered
Btrfs loaded, crc32c=crc32c-generic, zoned=yes
Key type encrypted registered
PM:   Magic number: 1:872:1009
RAS: Correctable Errors collector initialized.
md: Skipping autodetection of RAID arrays. (raid=autodetect will force)
qemu-system-x86_64: warning: 9p: degraded performance: a reasonable
high msize should be chosen on client/guest side (chosen msize is <=
8192). See https://wiki.qemu.org/Documentation/9psetup#msize for
details.
VFS: Mounted root (9p filesystem) readonly on device 0:20.
devtmpfs: mounted
Freeing unused decrypted memory: 2036K


Anybody see any clues ?

_______________________________________________
Kernelnewbies mailing list
Kernelnewbies@kernelnewbies.org
https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies

Re: VFS: Cannot open root device "(null)" or unknown-block(0, 0): error -2

[Valdis Klētnieks]

[-- Attachment #1.1: Type: text/plain, Size: 856 bytes --]

On Fri, 13 Aug 2021 18:08:38 -0600, jim.cromie@gmail.com said:

> then I added BTRFS_FS, since thats the host fs.
> of course it didnt work.

> VFS: Cannot open root device "(null)" or unknown-block(0,0): error -2
> Please append a correct "root=" boot option; here are the available partitions:
> Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
> CPU: 0 PID: 1 Comm: swapper Tainted: G        W         5.14.0-rc4+ #5

And it didn't find any usable btrfs (or other fs it knew about)

> Heres a working  kernel (if overlarge config)

> VFS: Mounted root (9p filesystem) readonly on device 0:20.

> Anybody see any clues ?

That '9p filesystem' should be a really big clue.  If that's the root fs that you
actually want to use, you need to do two things:

a) point at it with root=
b) have 9p filesystem support in the kernel

[-- Attachment #1.2: Type: application/pgp-signature, Size: 494 bytes --]

[-- Attachment #2: Type: text/plain, Size: 170 bytes --]

_______________________________________________
Kernelnewbies mailing list
Kernelnewbies@kernelnewbies.org
https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies

Re: VFS: Cannot open root device "(null)" or unknown-block(0, 0): error -2

[jim.cromie]

On Sat, Aug 14, 2021 at 6:21 PM Valdis Klētnieks
<valdis.kletnieks@vt.edu> wrote:
>
> On Fri, 13 Aug 2021 18:08:38 -0600, jim.cromie@gmail.com said:
>
> > then I added BTRFS_FS, since thats the host fs.
> > of course it didnt work.
>
> > VFS: Cannot open root device "(null)" or unknown-block(0,0): error -2
> > Please append a correct "root=" boot option; here are the available partitions:
> > Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
> > CPU: 0 PID: 1 Comm: swapper Tainted: G        W         5.14.0-rc4+ #5
>
> And it didn't find any usable btrfs (or other fs it knew about)
>
> > Heres a working  kernel (if overlarge config)
>
> > VFS: Mounted root (9p filesystem) readonly on device 0:20.
>
> > Anybody see any clues ?
>
> That '9p filesystem' should be a really big clue.  If that's the root fs that you
> actually want to use, you need to do two things:
>
> a) point at it with root=
> b) have 9p filesystem support in the kernel

the part you trimmed out contained this,
I think it means I had those particular modules built-in correctly

9pnet: Installing 9P2000 support
sched_clock: Marking stable (3466602654, 29303733)->(4050551166, -554644779)
Btrfs loaded, crc32c=crc32c-generic, zoned=no
9pnet_virtio: no channels available for device /dev/root

that last line is where it skipped the track.


so I really have to understand how virtme is doing its magic.
fwiw, it looks like its somewhere in this command line

./.virtme_mods/lib/modules/0.0.0
/usr/bin/qemu-system-x86_64 -fsdev
local,id=virtfs1,path=/,security_model=none,readonly,multidevs=remap
-device virtio-9p-pci,fsdev=virtfs1,mount_tag=/dev/root -fsdev
local,id=virtfs5,path=/usr/local/lib/python3.9/site-packages/virtme-0.1.1-py3.9.egg/virtme/guest,security_model=none,readonly,multidevs=remap
-device virtio-9p-pci,fsdev=virtfs5,mount_tag=virtme.guesttools
-machine accel=kvm:tcg -watchdog i6300esb -cpu host -parallel none
-net none -echr 1 -serial none -chardev
stdio,id=console,signal=off,mux=on -serial chardev:console -mon
chardev=console -vga none -display none -kernel
./arch/x86/boot/bzImage -append
'virtme_link_mods=/home/jimc/projects/lx/wk-next/builds/local-i915m/.virtme_mods/lib/modules/0.0.0
earlyprintk=serial,ttyS0,115200 console=ttyS0 psmouse.proto=exps
"virtme_stty_con=rows 24 cols 108 iutf8" TERM=xterm-256color
rootfstype=9p rootflags=version=9p2000.L,trans=virtio,access=any,msize=200M
raid=noautodetect ro nokaslr dynamic_debug.verbose=3
module.dyndbg=+pmf init=/bin/sh -- -c "mount -t tmpfs run /run;mkdir
-p /run/virtme/guesttools;/bin/mount -n -t 9p -o
ro,version=9p2000.L,trans=virtio,access=any,msize=104857600
virtme.guesttools /run/virtme/guesttools;exec
/run/virtme/guesttools/virtme-init"' -machine dump-guest-core=on -m 2G
--smp 3
Wrong EFI loader signature.
early console in extract_kernel

btw, I will go back and look for more 9P options.
and more VIRT options

thanks

_______________________________________________
Kernelnewbies mailing list
Kernelnewbies@kernelnewbies.org
https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies