Installation over serial via syslinux is unecessarily fiddly

[tomfitzhenry]

Installing nixos over serial via syslinux is fiddly, because:

• serial terminals aren't enabled by default on all stages
• on the boot stages that do have serial terminals enabled, they use inconsistent bitrates

Stage	Bitrate
BIOS	115200 (typically)
syslinux	38400 115200 (fixed in #58196)
Linux boot	serial terminal not enabled
Linux login	115200 (see stty)

In practice this means the user must either patch their image, or perform an installation while switching bitrates and adding serial terminals to kernel boot parameters.

We can make this smoother, by enabling serial on all stages, and ensuring they use the same bitrate.

• Change syslinux's bitrate to 115200: DONE syslinux: change serial bit rate to 115200 #58196
• Enable kernel serial console by default, by adding console=tty0 console=ttyS0,115200 to all boot entries.
  • PR pending: iso-image: add 'serial console' boot entry #91086

[tomfitzhenry]

My reading of the GRUB2 config suggests in GRUB2 the serial terminal isn't enabled by default:

nixpkgs/nixos/modules/installer/cd-dvd/iso-image.nix

Lines 311 to 317 in 2c07a08

	# Serial access is a must!
	submenu "" {return}
	submenu "Serial console=ttyS0,115200n8" --class serial {
	${grubMenuCfg}
	${buildMenuAdditionalParamsGrub2 config "console=ttyS0,115200n8"}
	}
	}

What's the rationale for this, and does this apply for syslinux?

[stale]

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[tomfitzhenry]

The remaining step is step 2.

Step 2: Enable kernel serial console by default, by adding console=tty0 console=ttyS0,115200 to all boot entries.

This is just a matter of copying the existing serial boot entry for GRUB ( https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/installer/cd-dvd/iso-image.nix#L325-L330 ) and adapting it to be a boot entry for syslinux: https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/installer/cd-dvd/iso-image.nix#L120-L146

I'll do this when I have time to test it.

[sneak]

This would be very useful to me, as I'm trying to install nixos via virt-install without vnc (headless, serial console).

[sneak]

Actually, I think just spawning a getty on the serial console after boot would be fine for my use case.

[tomfitzhenry]

sneak@, if you're able to test #91086 that'd be much appreciated. It should make installation easier for you.

[sneak]

Do you have an ISO I can download for that? Happy to test out.

[sneak]

FYI I started a thread about this here: https://discourse.nixos.org/t/feature-request-spawn-getty-on-serial-console-in-installer-iso/9195/2

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/feature-request-spawn-getty-on-serial-console-in-installer-iso/9195/3

[tomfitzhenry]

Do you have an ISO I can download for that? Happy to test out.

#91086 has instructions on building an ISO. See "I tested this change by building the image".

[tomfitzhenry]

Actually, I think just spawning a getty on the serial console after boot would be fine for my use case.

FWIW, I think the installer already does this. You just have to sit through a blank Linux boot process. If the boot succeeds, getty should open on the serial console. If the boot fails, getty won't get chance and it's hard to diagnose the boot failure since the Linux boot doesn't write to the serial console (that's what #91086 aims to fix).

[samueldr]

Stage-1 doesn't support multiple consoles yet.

#42255 was a first draft towards supporting this.

"Just" adding console=* here won't exactly help as only the last one will have the initial boot messages.

I mean to eventually get back to this problem and properly implement tests for bootlogd, in use in Mobile NixOS.

[sneak]

FWIW, I think the installer already does this.

It does not. VNCing in and running systemctl start serial-getty@ttyS0 makes it show up, though.

The only output from the system via serial is:

Loading /boot/bzImage... ok
Loading /boot/initrd...ok

...and that's it.