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

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Message-ID: <CAHk-=wjLytEj0Ccs1CsVPxmbLavf8Wk4ciDskhwy47rgyq7Oig@mail.gmail.com>
Date: Mon, 10 Jun 2024 17:50:31 -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 16:35, H. Peter Anvin <hpa@...or.com> wrote:
>
> ... which can be compacted down to a single instruction:
>
>     addq $bimm,%rax

We'll burn that bridge when  we get to it. I'm not actually seeing any
obvious for 32-bit immediates, except as part of some actual operation
sequence (ie you might have a similar hash lookup to the d_hash() one,
except using a mask rather than a shift).

When would you ever add a constant, except when that constant is an
address?  And those kinds of runtime constant addresses would always
be the full 64-bit - I don't think 32-bit is interesting enough to
spend any effort on.

(Yes, you can get 32-bit rip-address constants if you start doing
things like conditional link addresses, but that's what things like
static_call() is for - not this runtime constant code)

                Linus