[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 11:50 -0700, Richard Henderson wrote:
> On 10/06/2011 11:26 AM, Steven Rostedt wrote:
>  
> > 
> > That jmp to l_yes should easily be a two byte jump.
> 
> Until the compiler decides to re-order the code.  That's the problem --
> in the general case you do not know how far away the destination is really
> going to be.

Yeah, I was discussing this with Peter Zijlstra on IRC.

> 
> There are a couple of possibilities for improvement:
> 
> (1) Do as Jason suggests above and let the assembler figure out the size
> of the branch that is needed.  Without adding more data to __jump_table,
> you'll want to be extremely careful about checking the two pointers to 
> see what size branch has been installed.

Yeah, that could be done at patch time.


> 
> (2) Always reserve 5 bytes of space, but if the distance is small enough
> patch in a 2-byte jump.  That doesn't help with the icache footprint.

I don't think this one is worth it.

-- Steve


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