I have this problem where ACPI only recognizes 16 out of the 24
interrupts of my IOAPIC.

It's a VIA KT-400 chipset (Epox 8K9A3+), bios settings: ioapic on,
MPS-1.4. I'm using kernel 2.5.69 so my acpi-code is recent.

The relevant part of a dmesg of a acpi-2.5.69 boot:

ACPI: LAPIC_NMI (acpi_id[0x00] polarity[0x1] trigger[0x1] lint[0x1])
ACPI: IOAPIC (id[0x02] address[0xfec00000] global_irq_base[0x0])
IOAPIC[0]: Assigned apic_id 2
IOAPIC[0]: apic_id 2, version 3, address 0xfec00000, IRQ 0-23
Enabling APIC mode:  Flat.  Using 1 I/O APICs
init IO_APIC IRQs
 IO-APIC (apicid-pin) 2-0, 2-16, 2-17, 2-18, 2-19, 2-20, 2-21, 2-22, 2-23 not connected.
..TIMER: vector=0x31 pin1=2 pin2=0
number of MP IRQ sources: 16.
number of IO-APIC #2 registers: 24.

Notice how it first says IRQ 0-23 for the APIC, then later claims that
there are 16 MP IRQ sources. Unfortunately, my on-board network card is
wired to one of the 8 lost irq's, and after booting my network is dead.

If I boot a non-acpi kernel, things look like this:

I/O APIC #2 Version 17 at 0xFEC00000.
Enabling APIC mode:  Flat.  Using 1 I/O APICs
ENABLING IO-APIC IRQs
Setting 2 in the phys_id_present_map
...changing IO-APIC physical APIC ID to 2 ... ok.
init IO_APIC IRQs
 IO-APIC (apicid-pin) 2-0, 2-5, 2-10, 2-11, 2-20 not connected.
..TIMER: vector=0x31 pin1=2 pin2=0
number of MP IRQ sources: 24.
number of IO-APIC #2 registers: 24.

Then, there are 24 irq sources, and my network works fine.

Booting an acpi-kernel with commandline-argument 'pci=noacpi' hangs the
kernel after this line in the dmesg:

ide1 at 0x170-0x177,0x376 on irq 15

/proc/interrupts with acpi on:

           CPU0       
  0:    7451442    IO-APIC-edge  timer
  1:        403    IO-APIC-edge  i8042
  2:          0          XT-PIC  cascade
  4:          8    IO-APIC-edge  serial
  8:          4    IO-APIC-edge  rtc
  9:          0    IO-APIC-edge  acpi
 11:          0    IO-APIC-edge  VIA8233
 12:         63    IO-APIC-edge  i8042
 14:       9161    IO-APIC-edge  ide0
 15:       1906    IO-APIC-edge  ide1
 17:       1507   IO-APIC-level  ide2, ide4
 18:         53   IO-APIC-level  sym53c8xx
 19:          0   IO-APIC-level  EMU10K1

and with acpi off:

           CPU0       
  0:     465649    IO-APIC-edge  timer
  1:       2511    IO-APIC-edge  i8042
  2:          0          XT-PIC  cascade
  4:          8    IO-APIC-edge  serial
  8:          4    IO-APIC-edge  rtc
 12:         63    IO-APIC-edge  i8042
 14:      33566    IO-APIC-edge  ide0
 15:      14231    IO-APIC-edge  ide1
 17:       6509   IO-APIC-level  ide2, ide4
 18:         43   IO-APIC-level  sym53c8xx
 19:          0   IO-APIC-level  EMU10K1
 22:          0   IO-APIC-level  VIA8233
 23:      24486   IO-APIC-level  eth0

I'll attach a full dmesg for the acpi-kernel.
Googling didn't find anything to solve this, other than a tantalizing
hint this started between 2.5.44 and 2.5.44-ac1.

Thanks for your attention,
Jurriaan