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

[David Laight <david.laight.linux@gmail.com>]

On Tue, 22 Jul 2025 12:50:35 +0200
Oleg Nesterov <oleg@redhat.com> wrote:

> On 07/21, David Laight wrote:
> >
> > On Mon, 21 Jul 2025 15:04:22 +0200
> > Oleg Nesterov <oleg@redhat.com> wrote:
> >  
> > > Change mul_u64_u64_div_u64() to return ULONG_MAX if the result doesn't
> > > fit u64, this matches the generic implementation in lib/math/div64.c.  
> >
> > Not quite, the generic version is likely to trap on divide by zero.  
> 
> I meant that the generic implementation returns -1ul too if the result
> doesn't fit into u64.
> 
> > I think it would be better to always trap (eg BUG_ON(!div)).  
> 
> Well, I don't like adding a BUG_ON(), but OK.
> 
> > The trouble there is that (an ignored) ~(u64)0 is likely to cause another
> > arithmetic overflow with even more consequences.
> >
> > So I'm not at all sure what it should look like or whether 0 is a better
> > error return (esp for div == 0).  
> 
> I'm not sure either but x86/generic versions should be consistent. Let's
> discuss this and possibly change both implementations later?

My thought as well.
Getting both to agree is a start.

My latest thought is to add another parameter for the return value
when the result overflows or is infinity/NaN.
So the calling code can get 0, 1, ~0 (or any other 'safe' value) returned.
A special 'magic' value could be used to mean BUG().

> 
> > >  static inline u64 mul_u64_u64_div_u64(u64 a, u64 mul, u64 div)
> > >  {
> > > +	int ok = 0;
> > >  	u64 q;
> > >
> > > -	asm ("mulq %2; divq %3" : "=a" (q)
> > > -				: "a" (a), "rm" (mul), "rm" (div)
> > > -				: "rdx");
> > > +	asm ("mulq %3; 1: divq %4; movl $1,%1; 2:\n"  
> >
> > The "movl $1,%1" is a 5 byte instruction.
> > Better to use either 'incl' or get the constraints right for 'movb'  
> 
> Agreed, thanks,
> 
> > > +	if (ok)
> > > +		return q;
> > > +	WARN_ON_ONCE(!div);  
> >
> > I think you need to WARN for overflow as well as divide by zero.  
> 
> The generic implementation doesn't WARN... OK, I won't argue.

I've a set of patches I need to do a new version of.
I'll add a WARN_ON_ONCE() to the generic version.
I'll also put a copy of this patch in my set so that the later patches
will apply after this is applied without too much hastle.

> 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)

That needs to be "+q" (ok)

> 			: "a" (a), "rm" (mul), "rm" (div)
> 			: "rdx");
> 
> 		if (ok)
> 			return q;
> 		BUG_ON(!div);
> 		WARN_ON_ONCE(1);

I know there are are a lot of WARN_ON_ONCE(1) out there,
but maybe WARN_ON_ONCE("muldiv overflow") would be better?
(The linker will merge the strings).

	David

> 		return ~(u64)0;
> 	}
> 
> ?
> 
> Oleg.
>