APIC version and 8-bit APIC IDs

[Martin Wilck]

Hi William, hello everyone,

The MP_valid_apicid() function [arch/i386/kernel/mpparse.c] checks 
whether the APIC version field is >=20 in order to determine whether the 
CPU supports 8-bit physical APIC ids.

We currently have two modern processors oin our labs (Intel Xeon MP, AMD 
Dual-Core Opteron 875) for which this test is wrong because their APIC 
ids are both 16, but they _do_ support 8-bit APIC ids. This leads to 
erratic error messages and to valid CPUs not being detected.

Unfortunately I cannot tell why this is so, and what test should be used 
instead to make sure a CPU supports 8-bit APIC IDs.

The AMD BIOS and kernel developer's guide for Athlon64 and Opteron 
processors says

"When both ApicExtId and ApicExtBrdCst in the HyperTransport" 
Transaction Control Register are set, all 8 bits of APIC ID are used."

This refers to the TCR register. Reading that would require PCI 
configuration space access before the APICs are set up, I don't know if 
that's possible.
(http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/26094.PDF)

The Intel Manual simply says
"For the Pentium 4 and Intel Xeon Processors, the xAPIC specification 
extends the local APIC field to 8 bits". The CPUs we have are Xeon MP 
(family 15, model 4); their local APIC version is 16, and they do 
support 8-bit APIC-IDs.

I guess it's up to the Intel an AMD people to have a final word on this,
but the current implementation is clearly wrong for these latest CPU types.

Regards
Martin

--
Martin Wilck                Phone: +49 5251 8 15113
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