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

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Message-ID: <20250722105034.GA2845@redhat.com>
Date: Tue, 22 Jul 2025 12:50:35 +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 07/21, David Laight wrote:
>
> On Mon, 21 Jul 2025 15:04:22 +0200
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > Change mul_u64_u64_div_u64() to return ULONG_MAX if the result doesn't
> > fit u64, this matches the generic implementation in lib/math/div64.c.
>
> Not quite, the generic version is likely to trap on divide by zero.

I meant that the generic implementation returns -1ul too if the result
doesn't fit into u64.

> I think it would be better to always trap (eg BUG_ON(!div)).

Well, I don't like adding a BUG_ON(), but OK.

> The trouble there is that (an ignored) ~(u64)0 is likely to cause another
> arithmetic overflow with even more consequences.
>
> So I'm not at all sure what it should look like or whether 0 is a better
> error return (esp for div == 0).

I'm not sure either but x86/generic versions should be consistent. Let's
discuss this and possibly change both implementations later?

> >  static inline u64 mul_u64_u64_div_u64(u64 a, u64 mul, u64 div)
> >  {
> > +	int ok = 0;
> >  	u64 q;
> >
> > -	asm ("mulq %2; divq %3" : "=a" (q)
> > -				: "a" (a), "rm" (mul), "rm" (div)
> > -				: "rdx");
> > +	asm ("mulq %3; 1: divq %4; movl $1,%1; 2:\n"
>
> The "movl $1,%1" is a 5 byte instruction.
> Better to use either 'incl' or get the constraints right for 'movb'

Agreed, thanks,

> > +	if (ok)
> > +		return q;
> > +	WARN_ON_ONCE(!div);
>
> I think you need to WARN for overflow as well as divide by zero.

The generic implementation doesn't WARN... OK, I won't argue.
How about

	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)
			: "a" (a), "rm" (mul), "rm" (div)
			: "rdx");

		if (ok)
			return q;
		BUG_ON(!div);
		WARN_ON_ONCE(1);
		return ~(u64)0;
	}

?

Oleg.