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

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Message-ID: <20250722183853.GD2845@redhat.com>
Date: Tue, 22 Jul 2025 20:38:54 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: David Laight <david.laight.linux@...il.com>,
	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>,
	"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, H. Peter Anvin wrote:
>
> On July 22, 2025 10:58:08 AM PDT, Oleg Nesterov <oleg@...hat.com> wrote:
> >On 07/22, H. Peter Anvin wrote:
> >>
> >> On July 22, 2025 3:50:35 AM PDT, Oleg Nesterov <oleg@...hat.com> wrote:
> >> >
> >> >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.
> >>
> >> Maybe the generic version *should* warn?
> >
> >David is going to change the generic version to WARN.
> >
> >> As far as the ok output, the Right Way™ to do it is with an asm goto instead
> >> of a status variable; the second best tends to be to use the flags output.
> >
> >This is what I was going to do initially. But this needs
> >CONFIG_CC_HAS_ASM_GOTO_OUTPUT
> >
> >Oleg.
> >
>
> But that's what you want to optimize for, since that is all the modern compilers, even if you have to have two versions as a result.

Well, this 'divq' is slow anyway, I don't won't to add 2 versions.
Can we add the optimized version later if it really makes sense?

Oleg.