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From: Justin Piszcz <jpiszcz@lucidpixels.com>
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Subject: 2.6.22.1: Enabling IO-APIC = APIC error on CPU0: 40(40)
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I have an older MSI motherboard and when I enable the following option:

>>From lshw:
        description: Motherboard
        product: MS-6730

[*]   IO-APIC support on uniprocessors

I get the following errors in dmesg:

[  247.177372] APIC error on CPU0: 00(40)
[  247.556741] APIC error on CPU0: 40(40)
[  262.010412] APIC error on CPU0: 40(40)
[  262.765553] APIC error on CPU0: 40(40)
[  263.876481] APIC error on CPU0: 40(40)
[  264.191962] APIC error on CPU0: 40(40)
[  265.040566] APIC error on CPU0: 40(40)
[  267.959689] APIC error on CPU0: 40(40)
[  269.446029] APIC error on CPU0: 40(40)
[  269.853344] APIC error on CPU0: 40(40)
[  271.122825] APIC error on CPU0: 40(40)
[  271.287051] APIC error on CPU0: 40(40)
[  273.825237] APIC error on CPU0: 40(40)
[  274.373737] APIC error on CPU0: 40(40)
[  275.355775] APIC error on CPU0: 40(40)
[  275.982087] APIC error on CPU0: 40(40)
[  276.101181] APIC error on CPU0: 40(40)
[  276.173611] APIC error on CPU0: 40(40)
[  276.970970] APIC error on CPU0: 40(40)
[  277.561575] APIC error on CPU0: 40(40)
[  278.694486] APIC error on CPU0: 40(40)
[  279.353573] APIC error on CPU0: 40(40)
[  281.779398] APIC error on CPU0: 40(40)
[  283.625457] APIC error on CPU0: 40(40)
[  284.749322] APIC error on CPU0: 40(40)
[  284.983013] APIC error on CPU0: 40(40)
[  285.126394] APIC error on CPU0: 40(40)

I normally do not use this option (which also gets rid of the errors*1), 
but I am curious, why do these errors occur?  I remember them with the early
2.6.x series and they are still re-occurring in 2.6.22.1.

The weird part-- is when IO-APIC is disabled, I see this on boot:

IO-APIC DISABLED:

[   46.902712] XFS mounting filesystem md0
[   47.017322] spurious 8259A interrupt: IRQ7.
[   47.273009] Ending clean XFS mount for filesystem: md0

IO-APIC ENABLED:

[   46.851012] XFS mounting filesystem md0
[   47.103066] Ending clean XFS mount for filesystem: md0

* 1 Anytime I see a spurious interrupt on (usually older hardware) 
especially during file I/O this usually means corruption (from what I 
have seen) that is why I don't really want to trust anything on this host.

Anyone have any idea what's going on here?

Justin.