linux-kernel - Re: [RFC][PATCH 6/6] objtool,x86: Rewrite retpoline thunk calls

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Message-ID: <YDE9bmaO4tOZ/HWn@hirez.programming.kicks-ass.net>
Date:   Sat, 20 Feb 2021 17:48:46 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        pjt@...gle.com, mbenes@...e.cz, jgross@...e.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 6/6] objtool,x86: Rewrite retpoline thunk calls

On Sat, Feb 20, 2021 at 01:39:20AM +0100, Borislav Petkov wrote:
> On Fri, Feb 19, 2021 at 11:01:58PM +0100, Peter Zijlstra wrote:
> > We could, but it so happens Joerg is also wanting negative features. 
> 
> Juergen.

Argh! I should stick to jgross. Sorry.

> > So I was thikning that perhaps we can convince Boris they're not
> > really all that aweful after all :-)
> 
> Well, I'm not crazy about this, TBH - I totally agree with Josh:
> 
> "with objtool generating code, it's going to be a maze to figure out
> where the generated code is coming from"
> 
> and without a real good reason to do this, what's the point? I know,
> because we can. :-)

Well:

 - straight line execution is always better than a round-trip to
   somewhere else, no matter how trivial.
 - supposely EIBRS (yeah, I know, there's a paper out there) should
   result in no longer using retpolines.
 - I really, as in _REALLY_ don't want to do a CET enabled retpoline
 - IOW, retpolines should be on their way out (knock on wood)
 - doing this was fun :-)
 - this stuff was mostly trivial make work stuff I could do with a head
   full of snot and a headache.
 - if we had negative alternatives objtool doesn't need to actually
   rewrite code in this case. It could simply emit alternative entries
   and call it a day.
 - objtool already rewrites code
 - I have more cases for objtool to rewrite code (I'll see if I can
   rebase and post that this weekend -- no promises).
 - also https://lkml.kernel.org/r/20200625200235.GQ4781@hirez.programming.kicks-ass.net