linux-kernel - Re: [PATCH] x86/hweight: Fix and improve __arch

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Message-ID: <CAFULd4ZCc08kJU+3ZVdyWhO4s5fu0Y-RDPS-Y-_sPB1R0KrnoA@mail.gmail.com>
Date: Mon, 10 Mar 2025 21:35:42 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Borislav Petkov <bp@...en8.de>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/hweight: Fix and improve __arch_hweight{32,64}() assembly

On Mon, Mar 10, 2025 at 9:12 PM Borislav Petkov <bp@...en8.de> wrote:
>
> On Mon, Mar 10, 2025 at 09:08:04PM +0100, Uros Bizjak wrote:
> > a) Use ASM_CALL_CONSTRAINT to prevent inline asm that includes call
> > instruction from being scheduled before the frame pointer gets set
> > up by the containing function, causing objtool to print a "call
> > without frame pointer save/setup" warning.
>
> The other two are ok but this is new. How do you trigger this? I've never seen
> it in my randconfig builds...

It is not triggered now, but without this constraint, nothing prevents
the compiler from scheduling the insn in front of frame creation.

Uros.