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cat /proc/interrupts shows a bunch of IRQs such as NMI and LOC. The per-line comments in the output give clear explanation, but if they do not have a numeric IRQ number, how does the x86 CPU respond to them, in terms of entries in the Interrupt Descriptor Table?

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The non-numeric entries in /proc/interrupts correspond to arch-specific, non-device-related interrupts.

On x86, the IDT layout is described in arch/x86/include/asm/irq_vectors.h:

  • Vectors 0 ... 31 : system traps and exceptions - hardcoded events
  • Vectors 32 ... 127 : device interrupts
  • Vector 128 : legacy int80 syscall interface
  • Vectors 129 ... LOCAL_TIMER_VECTOR-1
  • Vectors LOCAL_TIMER_VECTOR ... 255 : special interrupts

The arch-specific interrupts are handled by IDT entries from 0 to 31 and from 129 to 255, with the local timer interrupt the first in the latter range. So when you see 0 in /proc/interrupts, it’s IDT entry 32; when you see NMI, it’s entry 2; etc. The IDT itself is set up in arch/x86/kernel/idt.c.

  • Thanks. I also found this file gives enumeration of system traps and exceptions /arch/x86/include/asm/traps.h. So does that mean that a standard linux on x86 can only support 127 - 31 = 96 external device interrupts? (I peeked into the same file on google's pixelbook and saw IRQs 201 for intel gpio and 207 for iwlwifi) – QnA Sep 10 '19 at 1:15
  • The IDT has been reworked several times; I described the current setup. Your Pixelbook might have been running an older kernel — in the past, fewer special interrupts were reserved, and device interrupts could go up to 200+. There are only 96 device interrupt entries in the IDT now, but that doesn’t limit the external device support much; PCI can share interrupts, the kernel supports hierarchical IRQ domains, etc. – Stephen Kitt Sep 10 '19 at 8:43

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