linux-kernel - Re: [PATCH 16/29] x86/bugs: Disable Retpoline when IBT

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Message-ID: <YhT6gthym/jCUME4@hirez.programming.kicks-ass.net>
Date:   Tue, 22 Feb 2022 16:00:18 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com
Subject: Re: [PATCH 16/29] x86/bugs: Disable Retpoline when IBT

On Fri, Feb 18, 2022 at 06:15:30PM -0800, Josh Poimboeuf wrote:

> This code is confusing, not helped by the fact that the existing code
> already looks like spaghetti.

I'd say that's an insult to spaghetti.

> Assuming IBT systems also have eIBRS (right?), I don't think the above
> SPECTRE_V2_CMD_{FORCE,AUTO} cases would be possible.

Virt FTW.. if I don't handle it, some idiot will create a virtual
machine that doesn't expose eIBRS but does do IBT just to spite me.

> AFAICT, if execution reached the retpoline_generic label, the user
> specified either RETPOLINE or RETPOLINE_GENERIC.

Only RETPOLINE_GENERIC;

> I'm not sure it makes sense to put RETPOLINE in the "silent" list.  If
> the user boots an Intel system with spectre_v2=retpoline on the cmdline,
> they're probably expecting a traditional retpoline and should be warned
> if that changes, especially if it's a "demotion".

too friggin bad as to expectations; retpoline == auto. Not saying that
makes sense, just saying that's what it does.

> In that case the switch statement isn't even needed.  It can instead
> just unconditinoally print the warning.
> 
> 
> Also, why "demote" retpoline to LFENCE rather than attempting to
> "promote" it to eIBRS?  Maybe there's a good reason but it probably at
> least deserves some mention in the commit log.

The current code will never select retpoline if eibrs is available.


The alternative is doing this in apply_retpolines(), but that might be
even more nasty.