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Message-ID: <548B24CC-2E58-4CC5-9025-950408BDCAA5@zytor.com>
Date: Tue, 22 Jul 2025 11:12:15 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Oleg Nesterov <oleg@...hat.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 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.

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