c9s: remove / override mitigation kargs

[vrutkovs]

When OKD SCOS is being installed it may use FCOS as a starting image (for clouds where we don't upload SCOS artifacts yet). This causes default FCOS kernel arguments to be retained - which means default mitigations=auto,nosmt are being kept and installation fails as only half the CPUs are being used.
For AWS the workaround is to use m6a.xlarge flavor where nosmt doesn't disable CPUs.

In OKD FCOS bootstrap automatically mixes in a MachineConfig which sets mitigations=off and all nodes boot without this karg after pivot. Ideally, we should find a solution to this in C9S manifest (and fall back to installer script if not possible)

[sdodson]

Or use any other instance type which is either not subject to nosmt disabling of SMT cores or has four cores available even after smt cores have been disabled. ie: m6i.xlarge only yields 2 cores and doesn't work but m6i.2xlarge would yield 4.

[LorbusChris]

This problem will go away once the installer switches to use SCOS bootimages, so there's no pivot that retains default kargs from FCOS.

More generally, this would be solved by coreos/rpm-ostree#3738

[travier]

We're likely not going to be able to fix that here and the end goal is to not use the FCOS image to bootstrap OKD-SCOS as this is a downgrade operation and we don't support those.

[cgwalters]

Just a side note: A bootc install --takeover flow would mean there's no "downgrade" because we entirely nuke and replace everything in the system (including the systemd journal, etc.). But, things like what image metadata flags are set are still relevant here.

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[LorbusChris]

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