Received-SPF: pass (google.com: domain of linux-kernel+bounces-209136-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.48.161 as permitted sender) client-ip=147.75.48.161;
Precedence: bulk
MIME-Version: 1.0
References: <20240608193504.429644-2-torvalds@linux-foundation.org>
 <20240610104352.GT8774@noisy.programming.kicks-ass.net> <20240610120201.GAZmbrOYmcA21kD8NB@fat_crate.local>
 <CAHk-=wgb98nSCvJ-gL42mt+jt6Eyp-0QSMJLovmAoJOkQ_G3gQ@mail.gmail.com> <71FE7A14-62F6-45D3-9BC4-BE09E06F7863@zytor.com>
In-Reply-To: <71FE7A14-62F6-45D3-9BC4-BE09E06F7863@zytor.com>
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 10 Jun 2024 17:50:31 -0700
Message-ID: <CAHk-=wjLytEj0Ccs1CsVPxmbLavf8Wk4ciDskhwy47rgyq7Oig@mail.gmail.com>
Subject: Re: [PATCH] x86: add 'runtime constant' infrastructure
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Borislav Petkov <bp@alien8.de>, Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>, 
	Thomas Gleixner <tglx@linutronix.de>, Rasmus Villemoes <linux@rasmusvillemoes.dk>, 
	Josh Poimboeuf <jpoimboe@kernel.org>, 
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, "the arch/x86 maintainers" <x86@kernel.org>, 
	linux-arch <linux-arch@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"

On Mon, 10 Jun 2024 at 16:35, H. Peter Anvin <hpa@zytor.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