linux-kernel - Re: [RFC][PATCH 0/9] Variable size jump

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Message-ID: <20191007120756.GE2311@hirez.programming.kicks-ass.net>
Date:   Mon, 7 Oct 2019 14:07:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, rostedt@...dmis.org,
        mhiramat@...nel.org, bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, jpoimboe@...hat.com, hjl.tools@...il.com
Subject: Re: [RFC][PATCH 0/9] Variable size jump_label support


In the failed thread Ingo posted:

>On Mon, Oct 07, 2019 at 01:26:06PM +0200, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > These here patches are something I've been poking at for a while, 
> > enabling jump_label to use 2 byte jumps/nops.
> > 
> > It _almost_ works :-/
> > 
> > That is, you can build some kernels with it (x86_64-defconfig for 
> > example works just fine).
> > 
> > The problem comes when GCC generates a branch into another section, 
> > mostly .text.unlikely. At that point GAS just gives up and throws a fit 
> > (more details in the last patch).
> > 
> > Aside from anyone coming up with a really clever GAS trick, I don't see 
> > how we can do this other than:
> 
> >  - use 'jmp' and get objtool to rewrite the text. Steven has earlier proposed
> >    something like that (using recordmcount) and Linus hated that.
> 
> As long as GCC+GAS correctly generates a 2-byte or 5-byte JMP depending 
> on the target distance, the objtool solution should work fine, shouldn't 
> it?
> 
> I can see the recordmcount solution sucking, it would depend on early 
> kernel patchery. But build time patchery is something we already depend 
> on, so assuming some objtool catastrophy it's a more robust solution, 
> isn't it?

IIRC the recordmcount variant from Steve was also rewriting JMP8 to NOP2
at build time.

I dug this here link out of my IRC logs:

  https://lore.kernel.org/lkml/1318007374.4729.58.camel@gandalf.stny.rr.com/

Looking at that, part of the reason might've been running yet another
tool, instead of having one tool do everything.