linux-kernel - Re: [PATCH v2] x86/hweight: Fix and improve __arch

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Message-ID: <Z9F7DoCQsIcf7MLe@gmail.com>
Date: Wed, 12 Mar 2025 13:16:14 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Uros Bizjak <ubizjak@...il.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2] x86/hweight: Fix and improve __arch_hweight{32,64}()
 assembly


* Uros Bizjak <ubizjak@...il.com> wrote:

> On Wed, Mar 12, 2025 at 1:09 PM Ingo Molnar <mingo@...nel.org> wrote:
> >
> >
> > * Uros Bizjak <ubizjak@...il.com> 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. This unconstrained scheduling might
> > > cause objtool to print a "call without frame pointer save/setup"
> > > warning. Current versions of compilers don't seem to trigger this
> > > condition, but without this constraint there's nothing to prevent
> > > the compiler from scheduling the insn in front of frame creation.
> > >
> > > b) Use asm_inline to instruct the compiler that the size of asm()
> > > is the minimum size of one instruction, ignoring how many instructions
> > > the compiler thinks it is. ALTERNATIVE macro that expands to several
> > > pseudo directives causes instruction length estimate to count
> > > more than 20 instructions.
> > >
> > > c) Use named operands in inline asm.
> >
> > I think this should be a 3-patch series. While it all modifies the same
> > code, each change could introduce separate issues, so it's better to be
> > individually bisectable IMO ...
> 
> Ok, will split and send separate series.

Thanks!

	Ingo