linux-kernel - Re: [PATCH v3 0/2] x86/math64: handle #DE in mul_u64_u64_div

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Message-ID: <20250817140726.223f8f72@pumpkin>
Date: Sun, 17 Aug 2025 14:07:26 +0100
From: David Laight <david.laight.linux@...il.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>, Dave Hansen
 <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, Ingo
 Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Thomas
 Gleixner <tglx@...utronix.de>, Li RongQing <lirongqing@...du.com>, Yu Kuai
 <yukuai3@...wei.com>, Khazhismel Kumykov <khazhy@...omium.org>, Jens Axboe
 <axboe@...nel.dk>, linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v3 0/2] x86/math64: handle #DE in mul_u64_u64_div_u64()

On Fri, 15 Aug 2025 18:40:09 +0200
Oleg Nesterov <oleg@...hat.com> wrote:

One of my 'idea patches' is to make mul_u64_u64_div_u64() a wrapper for
another function that takes in extra 'int *overflowed' parameter that is
set zero/non-zero for success/overflow.
The 'overflowed' parameter can either be a compile-time NULL or a
valid pointer.

So the x86-x64 asm implementation would use different code - you need
the 'jump around fail label' to write the ~0 return value to *overflowed.
The extra pointer check in the C version normal path may not be worth
worrying about (but the '*overflow = 0' could easily be inlined).

The typical use would be:
	quotient = mul_u64_u64_div_u64_overflow(..., &overflowed);
	if (quotient == ~0ull && overflowed)
		...
That will generate better code than returning 'overflowed' and the
quotient by reference.

Although I wonder how often ~0ull is a valid result?

	David