[Xen-devel] Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out

[Steven Rostedt <rostedt@xxxxxxxxxxx>]

On Thu, 2011-10-06 at 14:38 -0400, Jason Baron wrote:

> Right, its certainly doable, but I'm not sure its so simple, since we'll
> need a pass to eliminate the jumps - which can be keyed off the
> '__jump_table' section. 

Look at the code of scripts/recordmcount.c and friends.

It does two things.

1) find all the callers of mcount and make a section for it.

2) For those callers of mcount that is in sections that are not
whitelisted, and therefor will not be patched, to replace the call to
mcount with a nop.


We can use this code, or a copy of it, to do the same with jump_label.
Have the x86 jump_label be:


static __always_inline bool arch_static_branch(struct jump_label_key
*key)
{
        asm goto("1:"
                "jmp l_yes\n"
                ".pushsection __jump_table,  \"aw\" \n\t"
                _ASM_ALIGN "\n\t"
                _ASM_PTR "1b, %l[l_yes], %c0 \n\t"
                ".popsection \n\t"
                : :  "i" (key) : : l_yes);
        return false;
l_yes:
        return true;
}

Then have the record_jumplabel.c (or whatever it's called) find all the
jmps at run time, and convert them into the appropriate nop.

Then at runtime patching, the jumplabel code could figure out what size
jump it needs to replace it.

-- Steve



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