linux-kernel - Re: [PATCH] x86: add 'runtime constant' infrastructure

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Message-ID: <CAHk-=wjdsN=dH41MO+gASWZkexCgrwK6CGT=NvpA3xsVXEhxBw@mail.gmail.com>
Date: Mon, 10 Jun 2024 18:24:19 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, "the arch/x86 maintainers" <x86@...nel.org>, 
	linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH] x86: add 'runtime constant' infrastructure

On Mon, 10 Jun 2024 at 18:09, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Doing it in general is actually very very painful. Feel free to try -
> but I can almost guarantee that you will throw out the "Keep It Simple
> Stupid" approach and your patch will be twice the size if you do some
> "rewrite the whole instruction" stuff.
>
> I really think there's a fundamental advantage to keeping things simple.

I guess the KISS approach would be to have a debug mode that just adds
an 'int3' instruction *after* the constant. And then the constant
rewriting rewrites the constant and just changes the 'int3' into the
standard single-byte 'nop' instruction.

That wouldn't be complicated, and the cost would be minimal. But I
don't see it being worth it, at least not for the current use where
the unrewritten constant will just cause an oops on use.

                Linus