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

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Date:   Mon, 07 Oct 2019 10:44:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org
Cc:     peterz@...radead.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, mingo@...nel.org, namit@...are.com,
        hpa@...or.com, luto@...nel.org, ard.biesheuvel@...aro.org,
        jpoimboe@...hat.com, hjl.tools@...il.com
Subject: [RFC][PATCH 0/9] Variable size jump_label support

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:

 - get binutils/gas 'fixed', for this there's two possible approaches:

  * add some extra operators such that something like:

      .set disp %[l_yes] ~ 1b ? (%[l_yes] - (1b + 2)) : 128

    works, where the new '~' infix operator would indicate left and right hand
    operands are from the same section, and the ?: conditional operator would be
    added to make it all work.

  * add a 'fake' mnemonic for x86 that directly generates the right NOP:

      nojmp{.d8,d32} %target

    which would completely mirror how the existing 'jmp{.d8,d32} %target'
    works, except it would emit single instruction NOP2/NOP5.

 - use 'jmp' and get objtool to rewrite the text. Steven has earlier proposed
   something like that (using recordmcount) and Linus hated that.