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

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Date:   Mon, 7 Oct 2019 13:26:06 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, rostedt@...dmis.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


[ Sorry, fixed the Cc:lkml line. ]

* 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?

Thanks,

	Ingo