bring CONFIG_FORTIFY_SOURCE back from accidental removal...

Notice that it is not working anymore.

Rename pci.c into qemu_edu.c

Organize kernel_module readmes further

rootfs_overlay/qemu_edu.sh

@@ -0,0 +1,56 @@
+#!/bin/sh
+
+set -ex
+
+# Our modules does not the PCI device yet.
+lspci -k
+# => 00:04.0 Class 00ff: 1234:11e8 lkmc_pci
+
+# Interrupt counts before we generate our interrupts.
+cat /proc/interrupts
+
+# Setup.
+insmod /pci.ko
+/mknoddev.sh lkmc_pci
+
+# Shows that this module owns the PCI device.
+lspci -k
+# => 00:04.0 Class 00ff: 1234:11e8 lkmc_pci
+
+# Identifiction: just returns some fixed magic bytes.
+dd bs=4 status=none if=/dev/lkmc_pci count=1 skip=0 | od -An -t x1
+# => 010000ed
+
+# Negator. Sanity check that the hardware is getting updated.
+dd bs=4 status=none if=/dev/lkmc_pci count=1 skip=1 | od -An -t x1
+printf '\xF0\xF0\xF0\xF0' | dd bs=4 status=none of=/dev/lkmc_pci count=1 seek=1
+dd bs=4 status=none if=/dev/lkmc_pci count=1 skip=1 | od -An -t x1
+# => 0F0F0F0F
+
+# Factorial calculator.
+# Request interrupt when the computation is over.
+printf '\x80\x00\x00\x00' | dd bs=4 status=none of=/dev/lkmc_pci count=1 seek=8
+# factorial(0xC) = 0x1c8cfc00
+printf '\x0C\x00\x00\x00' | dd bs=4 status=none of=/dev/lkmc_pci count=1 seek=2
+# => irq_handler .*
+# Yes, we should use the interrupt to notify poll, but lazy.
+sleep 1
+dd bs=4 status=none if=/dev/lkmc_pci count=1 skip=2 | od -An -t x1
+dd bs=4 status=none if=/dev/lkmc_pci count=1 skip=8 | od -An -t x1
+# => 1c8cfc00
+
+# Manual IRQ raising.
+printf '\x04\x03\x02\x01' | dd bs=4 status=none of=/dev/lkmc_pci count=1 seek=24
+# => irq_handler .*
+sleep 1
+printf '\x08\x07\x06\x05' | dd bs=4 status=none of=/dev/lkmc_pci count=1 seek=24
+# => irq_handler .*
+sleep 1
+
+# Teardown.
+rm /dev/lkmc_pci
+rmmod pci
+
+# Interrupt counts after we generate our interrupts.
+# Compare with before.
+cat /proc/interrupts