Jump Label initialization

[Jeremy Fitzhardinge]

Hi all,

I'm trying to use the jump label machinery as part of the pv ticketlock
work I'm doing on x86.

The problem I'm having at the moment is that I do my spinlock setup in
smp_prepare_boot_cpu(), which happens before jump_label_init() gets
called, and so the latter goes and nops out all my enabled jump label key.

I'm experimenting at the moment with a patch to allow
jump_label_enable() to be called fairly early, and have that be
respected by jump_label_init().  I'm doing this by replacing
arch_jump_label_poke_text_early() with
arch_jump_label_transform_early(), which shares most of its code with
its non-early variant, except that it expects to run in a pre-SMP
environment.

Does this seem plausible? (I haven't tested it yet.)

The x86, mips and sparc patches are fairly simple; I forgot to look at
powerpc, and I didn't fully investigate s390.

While my current use-case is x86-specific, it seems generally useful to
make the jump_label machinery available as early as possible. I wonder
if you have any suggestions about how to handle this?

Thanks,
    J
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