How does VirtualBox handle interrupt for the guest Linux? - c

Question

I am practicing writing a simple keyboard driver in VirtualBox guest Linux. The problem is, my code just register an interrupt handler and print scancode to the log file. And I don't send those incoming scancode to any upper level codes, like Linux input core. After insmod, I can see those captured scancode using dmesg. But why my terminal still gets correct input? There should not be anything received by the terminal.
My code looks like this:
static int __init init_simple_keyboard_driver(void)
{
free_irq (IRQ_1, NULL);
return request_irq (IRQ_1, my_handler, ...);
}
static irqreturn_t my_handler(int irq, void *dev_id)
{
unsigned char scancode = get_scancode_from_port_0x60();
printk(...scancode...);
}
After insmod, I can see messages in the kernel log.
My free_irq call causes some messages like Can't free already freed IRQ. (I don't know why... It should not be freed already.)
atkbd driver complains that there is someone ask to handle IRQ_1 instead.
Those scancode can be correctly printed.
[The Most Weird One] The active console still gets correct keyboard input. Thus I can just perform a rmmod using this simple driver.
After rmmod, the guest Linux just dead because it can't receive any keyboard anymore.
Do you have any idea? Thank you!

Answer 1

The driver should not and could not un-register interrupt handler that is not registered by itself.
For preventing the original driver handle the keyboard interrupt, you can do ether:
1) return IRQ_HANDLED in your interrupt handler: This value indicates the interrupted is well handled and the linux kernel's interrupt processing mechanism would stop calling next interrupt handler. Or
2) clear input buffer in hardware, you can reference the original keyboard driver's code to know the status register and input buffer used during a keyboard hit event.