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VFS Cannot open root device with kernel 5.5.7 (GRUB2)
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Federile
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PostPosted: Wed Mar 04, 2020 5:16 pm    Post subject: VFS Cannot open root device with kernel 5.5.7 (GRUB2) Reply with quote

Hi,
So I wanted to change some configuration on my kernel 5.4.7, and I found out that by not having the source for that I had to download them again. At that point, I decided to go directly with 5.5.7 and after changing what I needed I reconfigured grub. But rebooting didn't work this time. I received a kernel panic with
Code:
VFS: Cannot open root device "nvme0n1p8" or unknown-block(0,0): error -6
please append a correct "root=" boot option

now I looked at the grub.cfg and the root partition is fine, but what I saw is that the Gentoo 5.5.7 option didn't have the initramfs. What should I do ?
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NeddySeagoon
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PostPosted: Wed Mar 04, 2020 5:26 pm    Post subject: Reply with quote

Federile,

Code:
VFS: Cannot open root device "nvme0n1p8" or unknown-block(0,0): error -6


NVMe support is missing from your new kernel.

grub did its thing and loaded your kernel.
NVMe support might be in an initrd you didn't load.
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Federile
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PostPosted: Thu Mar 05, 2020 5:34 pm    Post subject: Reply with quote

How can I do to give NVMe support? I looked at the configuration and the kernel has the module NVM Express block device
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NeddySeagoon
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PostPosted: Thu Mar 05, 2020 5:36 pm    Post subject: Reply with quote

Federile,

As <M>, in which case you need an initrd, or as <*> which means built into the kernel?
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Federile
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PostPosted: Thu Mar 05, 2020 5:46 pm    Post subject: Reply with quote

As <*> sorry, I thought after that it wasn't really a module.
Looking at what initramfs are I understood that has something to do with blobs, so I looked if this configuration had the blobs that I used before and it didn't.
So now I'm trying to recompile
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Federile
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PostPosted: Sat Mar 07, 2020 1:57 pm    Post subject: Reply with quote

So I tried adding the initramfs and I still have the same mistake, even if I think that genkernel didn't add the exact blobs that I needed. What should I do ?
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NeddySeagoon
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PostPosted: Sat Mar 07, 2020 4:22 pm    Post subject: Reply with quote

Federile,

A little expatiation of the boot process to aid your understanding of what happens and where your problem may be.

Something loads grub and grub gives you a menu. I'll skip the something as that works for you.
You make your choice at the grub menu then grub loads the kernel you selected and optionally, an initrd file.
Grub exits by jumping to the kernel start address.

At this point, grub is gone and its only the kernel and the optional initrd in memory. To keep things simple, lets say its just the kernel.
The kernel has to be able to mount the root filesystem unaided. There are kernel modules but they are located in /lib/modules on the root filesystem, so cannot be read to be used to mount the root filesystem. The kernel cannot read /etc/fstab to find the root filesystem until root is mounted. Hence the root= kernel parameter.

Your error
Code:
VFS: Cannot open root device "nvme0n1p8" or unknown-block(0,0): error -6
says that the kernel cannot reach /dev/nvme0n1p8 ...
Ahhh ... do you have root=nvme0n1p8 or root=/dev/nvme0n1p8 in grub.cfg ?

The (0,0) in unknown-block(0,0) is the kernel major,minor device numbers.

Please put your kernel .config and grub.cfg onto a pastebin site if its not a missing /dev/ in grub.cfg.
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NeddySeagoon

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