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

From: Don Slutz <dslutz@verizon.com>
To: Jan Beulich <JBeulich@suse.com>, Don Slutz <dslutz@verizon.com>
Cc: Jun Nakajima <jun.nakajima@intel.com>, Tim Deegan <tim@xen.org>,
	Kevin Tian <kevin.tian@intel.com>, Keir Fraser <keir@xen.org>,
	Ian Campbell <ian.campbell@citrix.com>,
	Stefano Stabellini <stefano.stabellini@eu.citrix.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	xen-devel@lists.xen.org, Eddie Dong <eddie.dong@intel.com>,
	Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>,
	Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [PATCH v4 01/16] xen: Add support for VMware cpuid leaves
Date: Wed, 17 Sep 2014 11:41:27 -0400	[thread overview]
Message-ID: <5419ABA7.50300@terremark.com> (raw)
In-Reply-To: <5416B48C0200007800034D7F@mail.emea.novell.com>


On 09/15/14 03:42, Jan Beulich wrote:
>>>> On 12.09.14 at 19:46, <dslutz@verizon.com> wrote:
>> On 09/12/14 05:49, Jan Beulich wrote:
>>>>>> On 11.09.14 at 21:49, <andrew.cooper3@citrix.com> 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
>
>