x86_64 Poweroff

info

This driver works only on x86_64.

We will build a driver that powers off QEMU.

Power Off

QEMU provides a virtual peripheral called ISA Debug Exit that maps an IO port. Writing a numeric value to this port will power off QEMU and return the value as an error code.

Attaching this device to QEMU is done by adding -device isa-debug-exit,iobase=0x10f4,iosize=0x04 to the command line. In this example, the debug device is mapped on port 0x10f4 and is 4 bytes long.

Simple Power Off Driver

The simplest driver that we can write is one that power off QEMU when it is loaded. We need to:

• reserve the port
• write to it using x86 assembly

Reserving the port region

note

The Kernel Rust API does not have (yet) and safe abstraction to reserve a port. This means that we have to use the C binding directly,

unsafe { bindings::request_region(start, len, name.as_char_ptr()) }

• start is the port number
• len is the gthe length of the port
• name is a name that will be displayed in /proc/ioports and is a C string char*

The function return NULL if the port mapping fails. In Rust, we can use the .is_null() function to check if the port reservation worked.

Writing to the port

Writing to the IO port is done using inline assembly. There are two possible syntaxes:

Kernel AT&T Syntax

// kernel API
use kernel::asm;
unsafe {
    asm!("
            outb %al, %dx
        ";
        in("dx") port as u16, 
        in("al") value as u8,
        options(noreturn)
    );
}

Intel Syntax

// core API
use core::arch::asm;
unsafe {
    asm!("
            out dx, al 
        ",
        in("dx") port as u16, 
        in("al") value as u8,
        options(intel_syntax, noreturn)
    );
}

warning

The kernel does not forbid writing to a port without reserving it, but this is bad practice, as we might override another driver's ports. if we cannot reserve a port, we just back off and fail to load the driver returning a EBUSY error.

Releasing the port region

If we have reserved a port region, we must make sure to release it when we do not need it anymore or when we unload the driver.

unsafe { bindings::release_region(start, len); }

Exercises

1. Write an empty module called PowerOff
2. Reserve the 0x10f4 port with 4 bytes long in the Module::init function.

• Print the contents of /proc/ioports to see if your port has been reserved
• Try reserving a port that already exists and fail to initialize the module

3. Release the 0x10f4 port with 4 bytes long in the Drop::drop function.