Linux Kernel Architecture

Chapter 14: Kernel Activities

Processing on IA-32 Systems

IA-32 requires slightly more work indo_IRQ, as the code flow diagram in Figure 14-10 shows. We first

suppose that a single page frame is used for the kernel stack, that is, 4 KiB are available per process for

the kernel. This is configured ifCONFIG_4KSTACKSis set. Recall that in this case a separate stack is used to

handle IRQ processing.

Switch stacks

Switch stacks back

No

Yes

do_IRQ

set_irq_regs

irq_enter

desc->handle_irq

desc->handle_irq

irq_exit

Stack switch necessary?

Figure 14-10: Code flow diagram fordo_IRQon IA-32

systems.

As in the AMD64 case, the functionsset_irq_regsandirq_enterare called with the same purpose as

before. The kernel must switch to the IRQ stack. The current stack can be obtained by calling the auxiliary

functioncurrent_thread_info, which delivers a pointer to thethread_infoinstance currently in use.

Recall from above that this information is in aunionwith the current stack. A pointer to the appropriate

IRQ-stack can be obtained fromhardirq_ctxas discussed above.

Two cases are possible:

1. The process is already using the IRQ stack because nested IRQs are processed. In this case,
   the kernel can be lazy — nothing needs to be done because everything is already set up.
   irq_desc[irq]->handle_irqcan be called to activate the ISR stored in the IRQ database.


2. The current stack is not the IRQ stack (curctx != irqctx), and a switch between both is
   required. In this case, the kernel performs the required low-level assembler operations to
   switch between the stacks, callsirq_desc[irq]->handle_irq, and switches the stacks back.

Note that in both cases the ISR is called directly and not via a detour overgeneric_handle_irqas on

AMD64 systems.

The remaining work is done in the same way as on AMD64 systems.irq_exithandles some account-

ing and activates SoftIRQs, andset_irq_regsrestores the register pointer to the state before the IRQ

happened.