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Re: Does the order of kernel parameters matter?

From: Salvatore Mazzarino
Subject: Re: Does the order of kernel parameters matter?
Date: Wed, 14 Oct 2020 19:54:03 +0000

It makes sense but I do not find correspondence.

I’m running the kernel 5.4.67 and according the COMMAND_LINE_SIZE value is 2048 https://github.com/torvalds/linux/blob/219d54332a09e8d8741c1e1982f5eae56099de85/arch/x86/include/asm/setup.h#L7 and my kernel parameter string is much less.

Am I missing anything?

On 14 October 2020 at 21:32:30, Jakob Bohm (jb-gnumlists@wisemo.com) wrote:

On 2020-10-14 20:39, Salvatore Mazzarino wrote:
I'm facing some interesting issues. Changing the order of kernel parameters passed to the `-append`, it changes the outcome.

Running my QEMU VM with the following works:

-append "root=/dev/disk/by-id/virtio-rootfs rootflags=rw flatcar.first_boot=1 tsc=reliable no_timer_check= rcupdate.rcu_expedited=1 i8042.direct=1 i8042.dumbkbd=1 i8042.nopnp=1 i8042.noaux=1 noreplace-smp= reboot=k console=hvc0 console=hvc1 cryptomgr.notests= net.ifnames=0 pci=lastbus=0"

However, if put the `root=` to the end then it does not work and the root volume is not found so not mounted.

So I'm now wondering. the order does it really matter? And if so there is any logic behind it? Any rules to follow? 

This happens generally with the Linux kernel, probably because the mechanism for
passing the options to the kernel has a character limit, causing the last of your
line to be thrown away.

I suggest looking at the kernel documentation to see if there is a limit and then
check if you exceed it.

As for qemu, I suspect that qemu uses a generic method of passing the string into
the VM, which has a much higher limit to support non-Linux kernels.


Jakob Bohm, CIO, Partner, WiseMo A/S.  https://www.wisemo.com
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