[RFC][PATCH 0/9] Variable size jump_label support

From: Peter Zijlstra
Date: Mon Oct 07 2019 - 07:24:00 EST


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.