console defaults for x86_64 qemu platform

[dustymabe]

With the work in coreos/coreos-assembler#2400 we'll soon be able to configure console defaults on each platform individually. After a notice period, the x86_64 qemu images will revert to no console= entry on the kernel command line, which means it will default to VGA console by default, which is what some users expect and will welcome the change.

For me (I suspect I'm not alone, but maybe), I use a lot of scripting to run various tests and I typically use the qemu qcow to pull that off. In a lot of cases I use the serial console output to debug, go through scrollback, etc.. In my cases I typically don't offer a graphical console to the machine at all (i.e. virt-install --graphics=none).

It would be nice if either the kernel would choose the serial console when there was no VGA console OR we set console defaults for QEMU that would fallback to serial console if VGA isn't available. From preliminary testing setting console=ttyS0 console=tty0 does that.

[bgilbert]

console=ttyS0 console=tty0 will still use ttyS0 as a secondary console, the way we're doing for VGA now. If we did that in the metal images, it'd still risk boot failures if the platform serial port doesn't work, and boot slowdowns if it does. But those concerns might not apply to QEMU.

[jlebon]

I think it might be worth trying to dig into the kernel auto-detection logic here some more and/or chat with the relevant kernel folks. I quoted the relevant kernel docs in #567 (comment) which seem to express exactly the semantics we'd want for kola/virt-install --graphics=none. But both @bgilbert and I tested this and we're not getting that result.

So either we're missing something, or the docs are wrong, or there's a bug in the kernel (non-exclusive :) ).

[bgilbert]

AFAICT if virtual consoles are enabled, they're always assumed to be present, so the kernel never falls back to serial. This is sort-of documented in Kconfig.

[bgilbert]

Testing shows that console=ttyS0 console=tty0 doesn't behave as desired. If there's no VGA console, the serial console stays secondary, so it only gets kernel messages.

[lucab]

From last meeting:

  * AGREED: for now, we will continue to add `console=tty0
    console=ttyS0,115200` to the QEMU image.  we will investigate having
    the initramfs e-shell print some info to secondary consoles, and
    possibly start additional shells on them.  (bgilbert, 17:29:11)

[dustymabe]

we will investigate having the initramfs e-shell print some info to secondary consoles

• 05core: print warnings to secondary consoles fedora-coreos-config#1769

and possibly start additional shells on them.

Looks like that is already the case today (see dracutdevs/dracut@32f68c1). At least I just verified on a qemu qcow2 with console=tty0 console=ttyS0,115200 that I get a shell on both consoles.

[dustymabe]

The fix for this went into testing stream release 36.20220605.2.0. Please try out the new release and report issues.

[dustymabe]

The fix for this went into stable stream release 36.20220605.3.0.