x86, irq: add IRQ layout comments

Describe the layout of x86 trap/exception/IRQ vectors and clean
up indentation and other small details.
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/include/asm/irq_vectors.h

@@ -3,6 +3,29 @@
 
 #include <linux/threads.h>
 
+/*
+ * Linux IRQ vector layout.
+ *
+ * There are 256 IDT entries (per CPU - each entry is 8 bytes) which can
+ * be defined by Linux. They are used as a jump table by the CPU when a
+ * given vector is triggered - by a CPU-external, CPU-internal or
+ * software-triggered event.
+ *
+ * Linux sets the kernel code address each entry jumps to early during
+ * bootup, and never changes them. This is the general layout of the
+ * IDT entries:
+ *
+ *  Vectors   0 ...  31 : system traps and exceptions - hardcoded events
+ *  Vectors  32 ... 127 : device interrupts
+ *  Vector  128         : legacy int80 syscall interface
+ *  Vectors 129 ... 237 : device interrupts
+ *  Vectors 238 ... 255 : special interrupts
+ *
+ * 64-bit x86 has per CPU IDT tables, 32-bit has one shared IDT table.
+ *
+ * This file enumerates the exact layout of them:
+ */
+
 #define NMI_VECTOR			0x02
 
 /*
@@ -27,6 +50,7 @@
  * Vectors 0x30-0x3f are used for ISA interrupts.
  */
 #define IRQ0_VECTOR			(FIRST_EXTERNAL_VECTOR + 0x10)
+
 #define IRQ1_VECTOR			(IRQ0_VECTOR +  1)
 #define IRQ2_VECTOR			(IRQ0_VECTOR +  2)
 #define IRQ3_VECTOR			(IRQ0_VECTOR +  3)