linux-kernel - Re: [PATCH] x86/alternatives: Fix alt_max

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Message-ID: <alpine.LSU.2.21.1710051445420.30711@wotan.suse.de>
Date:   Thu, 5 Oct 2017 14:52:19 +0200 (CEST)
From:   Michael Matz <matz@...e.de>
To:     Borislav Petkov <bp@...en8.de>
cc:     Mathias Krause <minipli@...glemail.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86-ml <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/alternatives: Fix alt_max_short macro to really be
 a max()

Hi,

On Thu, 5 Oct 2017, Borislav Petkov wrote:

> On Thu, Oct 05, 2017 at 02:35:33PM +0200, Mathias Krause wrote:
> > Note the "<"! ...comment is wrong, though the implementation works!
> 
> I know, I realized that when I looked at alternative-asm.h. Wanted to
> double-check it with Micha first.

Yeah, for bit-twiddling the result of arithmetic would need to be 
booleanized first, or alternatively the boolean operators be used in the 
first place.  So if '<' works then that's better in this context.

(In a different context, or in the same one there definitely was a problem 
with using '<', but I can't remember the details, it's too long ago we 
discussed about this; maybe it even was a problem only with some binutils 
versions.  So I'd suggest using the more obvious way until problems 
reoccur, and then document why exactly using relational ops was a problem 
;-) )

Ciao,
Michael.