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

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Message-ID: <20250818192831.0cc7a716@pumpkin>
Date: Mon, 18 Aug 2025 19:28:31 +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 Mon, 18 Aug 2025 14:39:01 +0200
Oleg Nesterov <oleg@...hat.com> wrote:

> 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?

I deliberately put this comment on 0/2 because it is 'future thought'.
I didn't want to delay the patch going in.

	David

> 
> 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
> >  
>