Kernel panic when using initramfs instead of initrd

[Ivan Nikolaev]

Hello everyone!
Briefly, I'm creating a dummy distribution which boots from CD (using 
VirtualBox for testing purposes) and lives in RAM. It's a very 
minimalistic kernel which has almost nothing enabled except from core 
facilities, virtual terminal, framebuffer and serial port.
I'm trying to use the "ultimate boot feature" - the initial RAM 
filesystem. Basically what I have is a plain simple directory with the 
following contents:

./proc
./sys
./dev
./etc
./etc/inittab
./etc/profile
./etc/passwd
./bin
./bin/init
./bin/sh
./bin/busybox

Obviously, at boot /bin/init is executed which is instructed by 
/etc/inittab to simply run a shell and stay happy.
EVERYTHING WORKS if I enable in the kernel some filesystem (like ext2 or 
squashfs) and create the initrd image of that type and put inside the 
contents of the directory. Isolinux then boots the kernel this way:

LABEL linux
     SAY Booting linux...
     KERNEL /vmlinuz
     APPEND root=/dev/ram0 initrd=/initrd.gz console=ttyS0,38400 vga=0x305

To make this work, I obviously enabled the "Initial RAM filesystem and 
RAM disk (initramfs/initrd) support" in "General setup" and the "RAM 
block device support" in "Device drivers - block devices" options.

Well, ensuring everything works, I tried to switch to initramfs. 
Disabled the "RAM block device support", the ext2 and squashfs 
filesystems and created the init cpio image with a command like this:

find | cpio -H newc -o | gzip -9 > initrd.gz

The boot arguments are the same. When I boot, what I have is a kernel 
panic with the following details:

List of all partitions:
No filesystem could mount root, tried:
Kernel panic - not syncing: VFS: Unable to mount root fs on 
unknown-block(0,0)

Here's my config file: http://pastebin.com/XVd8ZukU
Using linux v3.9.0

-- 
Ivan Nikolaev

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