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

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Date:   Mon, 7 Oct 2019 11:17:42 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, jpoimboe@...hat.com,
        hjl.tools@...il.com, Andrew Morton <akpm@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>,
        Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [RFC][PATCH 0/9] Variable size jump_label support

On Mon, 7 Oct 2019 13:26:06 +0200
Ingo Molnar <mingo@...nel.org> wrote:

> [ Sorry, fixed the Cc:lkml line. ]

/me joining the fun.

> 
> * 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?
> 

Actually, even back then I said that it would be best to merge all the
tools into one (I just didn't have the time to implement it), and then
we could pull this off. I have one of my developers working to merge
record-mcount into objtool now (there's been some patches floating
around).

Then with a single tool, it shouldn't be controversial.

-- Steve