Re: [Xen-devel] [PATCH v4 01/16] xen: Add support for VMware cpuid leaves

[Don Slutz <dslutz@xxxxxxxxxxx>]

On 09/15/14 03:42, Jan Beulich wrote:
> > > > On 12.09.14 at 19:46, <dslutz@xxxxxxxxxxx> wrote:
> > On 09/12/14 05:49, Jan Beulich wrote:
> > > > > > On 11.09.14 at 21:49, <andrew.cooper3@xxxxxxxxxx> wrote:
> > > > > +    case 0x10:
> > > > > +        /* (Virtual) TSC frequency in kHz. */
> > > > > +        *eax =  d->arch.tsc_khz;
> > > > > +        /* (Virtual) Bus (local apic timer) frequency in kHz. */
> > > > > +        *ebx = 1000000000ull / APIC_BUS_CYCLE_NS / 1000ull;
> > > > At least 1 pair of brackets please, especially as the placement of
> > > > brackets affects the result of this particular calculation.
> > > Or simply eliminate one of the divisions using
> > > "1000000ull / APIC_BUS_CYCLE_NS".
> > I am totally confused.  I am happy to go with Jan's version.
> >
> > The confusion is that I get the same answer all the ways I try.
> Hmm - this ...
>
> >           ebx1 = 1000000000ull / APIC_BUS_CYCLE_NS / 1000ull;
> >           ebx2 = (1000000000ull / APIC_BUS_CYCLE_NS) / 1000ull;
> >           ebx3 = 1000000000ull / (APIC_BUS_CYCLE_NS * 1000ull);
> ... clearly indicates the contrary: You converted to mutiplication
> here, when the respective possibility of putting parentheses here
> would have been
>
>           ebx3 = 1000000000ull / (APIC_BUS_CYCLE_NS / 1000ull);
>
> which I'm sure you agree won't produce the same result. But yes,
> the language implies parentheses this way
Yes, I agree.  Since that is not mathematical correct either.  This has 
the same issue
as "a - b -c" is the same as "(a - b) - c" but not "a - (b - c)". 
However the "order" can be
changed.  "a / b / c" and "a / c / b" is the same.

>           ebx2 = (1000000000ull / APIC_BUS_CYCLE_NS) / 1000ull;
>
> so the original expression without them was correct, just
> slightly ambiguous.
Ok, I see that preventing mistakes here can only help.
    -Don Slutz

> Jan

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