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

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Message-ID: <20210219220158.GD59023@worktop.programming.kicks-ass.net>
Date:   Fri, 19 Feb 2021 23:01:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     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 Fri, Feb 19, 2021 at 03:55:30PM -0600, Josh Poimboeuf wrote:
> On Fri, Feb 19, 2021 at 09:43:06PM +0100, Peter Zijlstra wrote:
> > Arguably it would be simpler to do the other way around, but
> > unfortunately alternatives don't work that way, we cannot say:
> > 
> > 	ALTERNATIVE "call __x86_indirect_thunk_\reg",
> > 		    "call *%reg", ~X86_FEATURE_RETPOLINE
> > 
> > That is, there is no negative form of alternatives.
> 
> X86_FEATURE_NO_RETPOLINE?

We could, but it so happens Joerg is also wanting negative features. So
I was thikning that perhaps we can convince Boris they're not really all
that aweful after all :-)