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

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Message-ID: <1317929321.4729.17.camel@gandalf.stny.rr.com>
Date:	Thu, 06 Oct 2011 15:28:39 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Richard Henderson <rth@...hat.com>
Cc:	Jason Baron <jbaron@...hat.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"David S. Miller" <davem@...emloft.net>,
	David Daney <david.daney@...ium.com>,
	Michael Ellerman <michael@...erman.id.au>,
	Jan Glauber <jang@...ux.vnet.ibm.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	peterz@...radead.org
Subject: Re: [PATCH RFC V2 3/5] jump_label: if a key has already been
 initialized, don't nop it out

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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