Re: APIC version and 8-bit APIC IDs

[Martin Wilck <martin.wilck@fujitsu-siemens.com>]

Hi Andi, hi 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.
> 
> Yes, it's broken. ... . Also it's only
> a sanity check for broken BIOS, and in this case it causes more problems
> than it solves.

We have another issue with the APIC IDs: the GET_APIC_ID and 
APIC_ID_MASK macros from the subarch code. In all subarchs except summit 
and es7000, these macros use a hard-coded mask 0x0F for the APIC ID. 
Thus, any APIC ID >15 will be truncated, leading to obscure errors.

We are wondering why these masks are there in the subarch code at all. 
After all, whether or not 8-bit APIC IDs are supported depends mainly on 
the CPU type used. Why wouldn't it possible to have a "default" 
architecture with APIC IDs > 15, if the CPUs allow it?

In other words: What would be broken if we just used an APIC ID mask of 
0xFF everywhere?

The current situation with MP_valid_apicid() on the one hand (masking 
the APIC ID as a function of local APIC version) and APIC_ID_MASK 
(masking the APIC as a function of subarch) on the other hand is 
inconsistent. A correct approach must take both CPU and architecture 
constraints into account, and use a CPU-type-dependent variable mask in 
the subarch code.

Regards
Martin

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Martin Wilck                Phone: +49 5251 8 15113
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