The easiest way to get to something extremely early is to parse it out
already in the setup code and pass in a flag in struct boot_params.
Otherwise, the kernel command line is present in memory and can be
copied or parsed in an ad hoc manner. For i386 it is copied in place
already in the assembly code -- for x86-64 it looks like we could
trivially move copy_bootdata until just past clear_bss().
-hpa
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