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

[Peter Zijlstra <peterz@infradead.org>]

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.