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SubjectRe: QUESTION: Network hangs with BP6 and 2.4.x kernels, hardware related?
Let's decode it:

> IO APIC #2......
> NR Log Phy Mask Trig IRR Pol Stat Dest Deli Vect:
> 12 0FF 0F 0 1 0 1 0 1 1 91
> 13 0FF 0F 0 1 1 1 0 1 1 99

IRR for interrupt 19 is set, that means the IO APIC has sent the
interrupt to a cpu but not yet received the corresponding EOI.

That bit is read only, so we can't set it to 0 to kick the io apic.

The Vector is 99, we must check that bit in the ISR, TMR and IRR of both

> ISR: all bits 0
> TMR: only bit 0x99 is set
> IRR: all bits 0

> ISR: all bits 0
> TMR: only bit 0x89 is set
> IRR: bit 0xfc and bit 0xef are set.

ISR is the in-server register, 0 means that the cpu is not processing an
interrupt right now.

TMR is the trigger mode registers, 1 means that the local apic should
send an EOI to the io apic when the cpu signals the EOI to the local

IRR is the list of pending interrupts:
0xef is the local timer interrupt,
0xfc is the reschedule interrupt
(see include/asm-i386/hw_irq.h)

These bits are also read only.

If you search the IO APIC documentation: number 29056601 - just search
with google. The local APIC is documented in the main cpu handbook (PPro
or later), in the chapter about multiple processor management

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