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

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Message-ID: <20230915075106.GA6787@noisy.programming.kicks-ass.net>
Date:   Fri, 15 Sep 2023 09:51:06 +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 Fri, Sep 15, 2023 at 09:46:47AM +0200, Peter Zijlstra wrote:
> On Wed, Sep 13, 2023 at 04:38:47PM +0200, Borislav Petkov wrote:
> 
> >   [ bp: Make labels unique and thus all sizing use unambiguous labels.
> >     Add more info. ]
> 
> > +#define __ALTERNATIVE(oldinstr, newinstr, ft_flags, n)			\
> > +	OLDINSTR(oldinstr, n)						\
> > +	ALTINSTR_ENTRY(ft_flags)					\
> > +	ALTINSTR_REPLACEMENT(newinstr)
> 
> > +#define ALTERNATIVE_2(oldinst, newinst1, flag1, newinst2, flag2)	\
> > +	__ALTERNATIVE(ALTERNATIVE(oldinst, newinst1, flag1),		\
> > +		    newinst2, flag2, 1)
> 
> > +#define ALTERNATIVE_3(oldinst, newinst1, flag1, newinst2, flag2,	\
> > +		      newinst3, flag3)					\
> > +	__ALTERNATIVE(ALTERNATIVE_2(oldinst, newinst1, flag1, newinst2, flag2), \
> > +		    newinst3, flag3, 2)
> 
> 
> So I see what you did with that @n argument, but urgh, do we really need
> this? I mean, it just makes things harder to use and it doesn't actually
> fix anything.. :/

That is, if we can magic this using __COUNTER__ without a user interface
penalty, then sure. But the last time I tried that I failed utterly and
ended up with labels like:

  .Lalt_old___COUNTER__:

no matter how many layers of CPP macro eval I stuck in it. So clearly I
wasn't having a good day ....