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

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Message-ID: <20250722132147.GB2845@redhat.com>
Date: Tue, 22 Jul 2025 15:21:48 +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 12:50:35 +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), "+r" (ok)
>
> That needs to be "+q" (ok)

Thanks... I will never understand the asm constraints.

> > 		if (ok)
> > 			return q;
> > 		BUG_ON(!div);
> > 		WARN_ON_ONCE(1);
>
> I know there are are a lot of WARN_ON_ONCE(1) out there,
> but maybe WARN_ON_ONCE("muldiv overflow") would be better?
> (The linker will merge the strings).

OK. If you are fine with this version I'll send V2.

	/*
	 * Returns ULONG_MAX when the result doesn't fit u64.
	 */
	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");
		return ~(u64)0;
	}

Oleg.