Location of interrupt handling code in Linux kernel for x86 architecture

Question

I am doing so research trying to find the code in the Linux kernel that implements interrupt handling; in particular, I am trying to find the code responsible for handling the system timer.

According to http://www.linux-tutorial.info/modules.php?name=MContent&pageid=86

The kernel treats interrupts very similarly to the way it treats exceptions: all the general >purpose registers are pushed onto the system stack and a common interrupt handler is called. >The current interrupt priority is saved and the new priority is loaded. This prevents >interrupts at lower priority levels from interrupting the kernel while it handles this >interrupt. Then the real interrupt handler is called.

I am looking for the code that pushes all of the general purpose registers on the stack, and the common interrupt handling code.

At least pushing the general purpose registers onto the stack is architecture independent, so I'm looking for the code that is associated with the x86 architecture. At the moment I'm looking at version 3.0.4 of the kernel source, but any version is probably fine. I've gotten started looking in kernel/irq/handle.c, but I don't see anything that looks like saving the registers; it just looks like it is calling the registered interrupt handler.

Answer 1

The 32-bit versions are in arch/i386/kernel/entry_32.S, the 64-bit versions in entry_64.S. Search for the various ENTRY macros that mark kernel entry points.

Answer 2

I am looking for the code that pushes all of the general purpose registers on the stack

Hardware stores the current state (which includes registers) before executing an interrupt handler. Code is not involved. And when the interrupt exits, the hardware reads the state back from where it was stored.

Now, code inside the interrupt handler may read and write the saved copies of registers, causing different values to be restored as the interrupt exits. That's how a context switch works.

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On x86, the hardware only saves those registers that change before the interrupt handler starts running. On most embedded architectures, the hardware saves all registers. The reason for the difference is that x86 has a huge number of registers, and saving and restoring any not modified by the interrupt handler would be a waste. So the interrupt handler is responsible to save and restore any registers it voluntarily uses.

See Intel® 64 and IA-32 Architectures Software Developer’s Manual, starting on page 6-15.