linux-kernel - Re: [PATCH v2 10/11] x86/alternatives: Simplify ALTERNATIVE

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Message-ID: <20230912092709.GE35261@noisy.programming.kicks-ass.net>
Date:   Tue, 12 Sep 2023 11:27:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, David.Kaplan@....com,
        Andrew.Cooper3@...rix.com, jpoimboe@...nel.org,
        gregkh@...uxfoundation.org, nik.borisov@...e.com
Subject: Re: [PATCH v2 10/11] x86/alternatives: Simplify ALTERNATIVE_n()

On Sun, Sep 10, 2023 at 04:42:27PM +0200, Borislav Petkov wrote:
> On Sat, Sep 09, 2023 at 11:25:54AM +0200, Peter Zijlstra wrote:
> > So what you end up with is:
> > 
> > 661:
> >   "one byte orig insn"
> >   "one nop because alt1 is 2 bytes"
> >   "one nop because alt2 is 3 bytes"
> > 
> > right?
> 
> Right.
> 
> > This becomes more of a problem with your example above where the
> > respective lengths are 0, 5, 16. In that case, when you patch 5, you'll
> > leave 11 single nops in there.
> 
> Well, I know what you mean but the code handles that gracefully and it
> works. Watch this:

Aah, because we run optimize_nops() for all alternatives, irrespective
of it being selected. And thus also for the longest and then that'll fix
things up.

OK, let me check on objtool.