Re: Re: [PATCH] libxc: remove CPUID core information mangling

[Andre Przywara <andre.przywara@amd.com>]

Keir Fraser wrote:
> Ah yes, I agree.
> 
> 
> On 25/08/2010 16:53, "Huang2, Wei" <Wei.Huang2@amd.com> wrote:
> 
>> OK. BTW, the old way seems wrong. The correct implementation should be
>> (((regs[2] & 0xf000u) >> 12) + 1) << 12.
>>
>> -----Original Message-----
>> From: Keir Fraser [mailto:keir.fraser@eu.citrix.com]
>> Sent: Wednesday, August 25, 2010 10:39 AM
>> To: Huang2, Wei; Przywara, Andre; Nitin Kamble
>> Cc: xen-devel
>> Subject: Re: [Xen-devel] Re: [PATCH] libxc: remove CPUID core information
>> mangling
>>
>> I meant it should remain the old way, since HVM virtual APIC IDs are
>> vcpu_id*2.
I agree, that seems to be best way for the time being. Although this 
value is actually meant to tell different processors apart, so I guess 
it needs a revisit later.

FYI:
Real machines use different ways to assign APIC-IDs, for example my 
4-way Magny-Cours (4*12 cores) has this:
0x00-0x02: used for I/O-APICs, (could be 4-bit constrained)
     -0x0f: reserved for IOAPICs
0x10-0x1b: LAPIC-IDs for cores from the 1st processor
0x20-0x2b: LAPIC-IDs for cores from the 2nd processor
0x30-0x3b: LAPIC-IDs for cores from the 3rd processor
0x40-0x4b: LAPIC-IDs for cores from the 4th processor
The 80000008:ECX[15:12] value is 4, which means the lower 4 bits of the 
LAPIC ID indicate the core number within each package.
Obviously this scheme does not fit the Xen one's.

Thanks Wei for spotting the calculation error!

Regards,
Andre.

>>
>>  -- Keir
>>
>> On 25/08/2010 16:28, "Huang2, Wei" <Wei.Huang2@amd.com> wrote:
>>
>>> Hi Keir,
>>>
>>> Do you mean that we should leave 80000008:ECX[15:12] as zero or in old way
>>> (i.e. (regs[2] & 0xf000u) + 1))? These bits can't be zero, unless we want to
>>> use legacy method in multi-core calculation.
>>>
>>> -Wei
>>>
>>> ========
>>> I think you shouldn't change handling of 80000008:ECX[15:12] since that does
>>> explicitly refer to APIC ID arrangement. The rest of your changes could be
>>> correct as far as I can tell from the reference manuals.
>>>


-- 
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 448-3567-12