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

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Message-ID: <20250818123900.GB18626@redhat.com>
Date: Mon, 18 Aug 2025 14:39:01 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: David Laight <david.laight.linux@...il.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()

David,

We had a lengthy discussion and you have already acked this fix.

I thought that we agreed on that a) we need to fix the problem first
and b) x86 version should be consistent with the generic implementation
regarding ~0ull on overflow.

Can we finally merge this fix, then discuss the possible improvements
and possibly change both implementation?

Oleg.

On 08/17, David Laight wrote:
>
> 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
>