Re: [PATCH v3 0/2] x86/math64: handle #DE in mul_u64_u64_div_u64()

[Oleg Nesterov]

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@xxxxxxxxxx> 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
>