Linux Kernel Architecture

Chapter 14: Kernel Activities

Calling the Flow HandlerRoutine

How the flow handler routines are called differs from architecture to architecture; in the following,

how this is done is discussed for AMD64 and IA-32. Additionally, we also examine the old han-

dler mechanism that was the default before the IRQ subsystem rewrite, and is still used in some

places.

Processing on AMD64 Systems

Let us first turn our attention to howdo_IRQis implemented on AMD64 systems. This variant is simpler

as compared to IA-32, and many other modern architectures use a similar approach. The code flow

diagram is shown in Figure 14-9.

do_IRQ

set_irq_regs

set_irq_regs

irq_enter

generic_handle_irq

irq_exit

Figure 14-9: Code flow diagram

fordo_IRQ.on AMD64 systems.

The prototype of the function is as follows:

arch/x86/kernel/irq_64.c

asmlinkage unsigned int do_IRQ(struct pt_regs *regs)

The low-level assembler code is responsible to passthe current state of the register set to the function,

and the first task ofdo_IRQis to save a pointer to them in a global per-CPU variable usingset_irq_regs

(the old pointer that was active before the interruptoccurred is preserved for later). Interrupt handlers

that require access to the register set can access them from there.

irq_enteris then responsible to update some statistics; for systems with dynamic ticks, the global

jiffiestime-keeping variable is updated if the system has been in a tickless state for some time

(more about dynamic ticks follows in Section 15.5.). Calling the ISRs registered for the IRQ in

question is then delegated to the architecture-independentfunctiongeneric_handle_irq, which calls

irq_desc[irq]->handle_irqto activate the flow control handler.

irq_exitis then responsible for some statistics bookkeeping, but also calls (assuming the kernel is not

still in interrupt mode because it is processing a nested interrupt)do_softirqto service any pend-

ing software IRQs. This mechanism is discussed in more detail in Section 14.2. Finally, another call to

set_irq_regsrestores the pointer tostruct regsto the setting that was active before the call. This

ensures that nested handlers work correctly.