[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 14:10 -0400, Jason Baron wrote:

> > Looks like jmp2 is about 5% faster than jmp5 on Sandybridge with this
> > benchmark.
> > 
> > But insignificant difference on Nehalem.
> > 
> >     J
> 
> It would be cool if we could make the total width 2-bytes, when
> possible.  It might be possible by making the initial 'JUMP_LABEL_INITIAL_NOP'
> as a 'jmp' to the 'l_yes' label. And then patching that with a no-op at boot
> time or link time - letting the compiler pick the width. In that way we could
> get the optimal width...

Why not just do it?

jump_label is encapsulated in arch_static_branch() which on x86 looks
like:

static __always_inline bool arch_static_branch(struct jump_label_key *key)
{
        asm goto("1:"
                JUMP_LABEL_INITIAL_NOP
                ".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;
}


That jmp to l_yes should easily be a two byte jump.

If not I'm sure it would be easy to catch it before modifying the code.
And then complain real loudly about it.

-- Steve



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