linux-kernel - Re: [PATCH next 10/14] bits: Fix assmebler expansions of GENMASK_Uxx() and BIT

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Message-ID: <aYgDlpdXWqYvj3LN@yury>
Date: Sat, 7 Feb 2026 22:31:34 -0500
From: Yury Norov <ynorov@...dia.com>
To: david.laight.linux@...il.com
Cc: Nathan Chancellor <nathan@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, Yury Norov <yury.norov@...il.com>,
	Lucas De Marchi <lucas.demarchi@...el.com>,
	Jani Nikula <jani.nikula@...el.com>,
	Vincent Mailhol <mailhol.vincent@...adoo.fr>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH next 10/14] bits: Fix assmebler expansions of
 GENMASK_Uxx() and BIT_Uxx()

On Wed, Jan 21, 2026 at 02:57:27PM +0000, david.laight.linux@...il.com wrote:
> From: David Laight <david.laight.linux@...il.com>
> 
> The assembler only supports one type of signed integers, so expressions
> using BITS_PER_LONG (etc) cannot be guaranteed to be correct.
> 
> Use ((2 << (h)) - (1 << (l))) for all assembler GENMASK() expansions and
> add definitions of BIT_Uxx() as (1 << (nr)).
> 
> Note that 64bit results are (probably) only correct for 64bit builds
> and 128bits results will never be valid.

And this important note will sink in git history.
 
> Signed-off-by: David Laight <david.laight.linux@...il.com>

This has been discussed in details when those GENMASK_Uxx() were
introduced. Assembler doesn't support C types, and can't provide any
guarantees. It may only confuse readers when they see something like
GENMASK_U8() in the assembler code, and there's nothing on behalf of
that declaration to enforce the limitation. 

That's why we didn't add fake C types support in the assembler. Unless
we find a way to enforce C types capacity in assembler(s), let's keep
those macros C-only.

Thanks,
Yury