linux-kernel - Re: [PATCH] x86/math64: handle #DE in mul_u64_u64_div

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Message-ID: <20250723093825.GA12884@redhat.com>
Date: Wed, 23 Jul 2025 11:38:25 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: David Laight <david.laight.linux@...il.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Li,Rongqing" <lirongqing@...du.com>,
	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	x86@...nel.org
Subject: Re: [PATCH] x86/math64: handle #DE in mul_u64_u64_div_u64()

On 07/22, David Laight wrote:
>
> On Tue, 22 Jul 2025 15:21:48 +0200
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > 	static inline u64 mul_u64_u64_div_u64(u64 a, u64 mul, u64 div)
> > 	{
> > 		char ok = 0;
> > 		u64 q;
> >
> > 		asm ("mulq %3; 1: divq %4; movb $1,%1; 2:\n"
> > 			_ASM_EXTABLE(1b, 2b)
> > 			: "=a" (q), "+q" (ok)
> > 			: "a" (a), "rm" (mul), "rm" (div)
> > 			: "rdx");
> >
> > 		if (ok)
> > 			return q;
> > 		BUG_ON(!div);
> > 		WARN_ONCE(1, "muldiv overflow.\n");
>
> I wonder what WARN_ON_ONCE("muldiv overflow") outputs?

Well, it outputs "muldiv overflow." ;) So I am not sure it is better
than just WARN_ON_ONCE(1).

> Actually, without the BUG or WARN you want:
> 	u64 fail = ~(u64)0;
> then
> 	incq $1 ... "+r" (fail)
> and finally
> 	return q | fail;
> to remove the conditional branches from the normal path
> (apart from one the caller might do)

I was thinking about

	static inline u64 mul_u64_u64_div_u64(u64 a, u64 mul, u64 div)
	{
		u64 q;

		asm ("mulq %2; 1: divq %3; jmp 3f; 2: movq $-1,%0; 3:\n"
			_ASM_EXTABLE(1b, 2b)
			: "=a" (q)
			: "a" (a), "rm" (mul), "rm" (div)
			: "rdx");

		return q;
	}

to remove the conditional branch and additional variable. Your version
is probably beterr... But this is without WARN/BUG.

So, which version do you prefer?

Oleg.