fix: disable Kexec Handover to free up ~128MiB of RAM

[lowjoel]

What kind of change does this PR introduce?

fix: disable Kexec Handover to free up ~128MiB of RAM

Required for KUBE-1370.

What is the current behavior?

On Linux 7.0 kernels kho is enabled by Ubuntu:

ubuntu@db-jrskfeoczfnalrxisytn:~$ sudo dmesg | grep -i 'cma-'
[  +0.000101] Memory: 545236K/819200K available (27136K kernel code, 6266K rwdata, 20296K rodata, 15232K init, 1301K bss, 233864K reserved, 32768K cma-reserved)

This means that we're reserving even more memory than on 6.8.0:

ubuntu@db-nfkaciskftvcorhjdwao:~$  cat /proc/cmdline
BOOT_IMAGE=/vmlinuz-6.8.0-106-generic root=PARTUUID=6555485d-34e0-4488-9101-ad42900d9ace ro console=tty1 console=ttyAMA0
ubuntu@db-nfkaciskftvcorhjdwao:~$ sudo dmesg | grep -i 'cma-'
[    0.000000] Memory: 701468K/819200K available (23488K kernel code, 5208K rwdata, 16440K rodata, 12288K init, 1233K bss, 84964K reserved, 32768K cma-reserved)

What is the new behavior?

ubuntu@db-jrskfeoczfnalrxisytn:~$  cat /proc/cmdline
BOOT_IMAGE=/vmlinuz-7.0.0-30-generic root=PARTUUID=f257fc73-df70-4995-8eb6-953e15761105 ro console=tty1 console=ttyAMA0 kho=off panic=-1
ubuntu@db-jrskfeoczfnalrxisytn:~$  sudo dmesg | grep 'cma-'
[    0.003953] Memory: 682576K/819200K available (27136K kernel code, 6266K rwdata, 20296K rodata, 15232K init, 1301K bss, 96588K reserved, 32768K cma-reserved)

Additional context

kho reserves memory to be handed over to the kexec'ed kernel; on memory
constrained instances this is a significant amount of memory. Disable
kho since we don't use kexec to reboot anyway.

[mmlb]

Suggested change

	when:
	- qemu
	when: qemu

but actually why not for all builds?

[lowjoel]

I don't think we use kexec in v2; but I wanted to keep this for v3 only. If we are ok with this in v2 I can remove the restriction. Let me test that it does as it says on v3 first.

[lowjoel]

OK I've tested this with 17.6.1.175-lowjoel, kho=off is set in cmdline and reserved memory is back to ~96MiB. I've removed the qemu restriction; @mmlb could you have a review please?

[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved issues were identified.

Review effort: Lite
Findings: None

What changed in this PR

Disables Kexec Handover to reclaim approximately 128 MiB of RAM on constrained instances.

Changes:

• Adds kho=off to GRUB kernel parameters.
• Registers handlers to regenerate GRUB configuration.

File	Description
ansible/​tasks/​setup-system.yml	Writes the KHO-disabled GRUB configuration.
ansible/​playbook.yml	Registers handler imports.
ansible/​handlers/​update-grub.yml	Regenerates GRUB configuration.
ansible/​handlers/​main.yml	Imports the GRUB handler.

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[mmlb]

Nice find!