linux-kernel - Re: [tip: objtool/core] jump_label, x86: Allow short NOPs

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Message-ID: <YKS2oX/PCfp4NQ8V@hirez.programming.kicks-ass.net>
Date:   Wed, 19 May 2021 08:56:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Ingo Molnar <mingo@...nel.org>, x86@...nel.org,
        willy@...radead.org, masahiroy@...nel.org, michal.lkml@...kovi.net
Subject: Re: [tip: objtool/core] jump_label, x86: Allow short NOPs

On Tue, May 18, 2021 at 07:44:11PM -0500, Josh Poimboeuf wrote:

> I'm not exactly thrilled that objtool now has the power to easily brick
> a system :-/  Is it really worth it?

The way I look at it is that not running objtool is a bug either way,
bricking a system is ofcourse a somewhat more drastic failure mode than
missing ORC info for example, but neither are good.

As to worth, about half the jump labels are shorter now, this reduces I$
pressure on hot paths. Any little thing to offset the ever increasing
bulk seems like a good thing to me. But yes, it would be nice if the
assemblers wouldn't suck so bad and this wouldn't need objtool :/ But
I've tried poking the tools guys and they don't really seem interested
:-(

Also, only dirty builds are affected here; clean builds (always
recommended afaik, because dep trouble isn't unheard of) are fine.

> Anyway, here's one way to fix it.  Maybe Masahiro has a better idea.

Thanks! lemme go read up on this magic :-)