feat(vmm): exit the VMM on kbd reset (#12)

Currently, when killing the `init` process, the VMM hangs because it is
unable to handle the kernel trying to reboot the machine.

On an x86 PC platform, it is possible to reset the CPU by [sending byte
`0xFE` to the keyboard controller command register](https://wiki.osdev.org/%228042%22_PS/2_Controller#CPU_Reset).
This is actually part of the emergency reboot procedure of [the Linux kernel](https://elixir.bootlin.com/linux/v6.8.2/source/arch/x86/kernel/reboot.c#L627).

This adds a bit of code to handle such a write operation by exiting the
VMM process.

Signed-off-by: Kuruyia <github@kuruyia.net>

src/vmm/src/core/cpu/mod.rs

@@ -5,8 +5,8 @@ use crate::core::devices::serial::{LumperSerial, SERIAL_PORT_BASE, SERIAL_PORT_L
 use kvm_bindings::{kvm_fpu, kvm_regs, CpuId};
 use kvm_ioctls::{VcpuExit, VcpuFd, VmFd};
 use std::convert::TryInto;
-use std::io;
 use std::sync::{Arc, Mutex};
+use std::{io, process};
 use std::{result, u64};
 use tracing::{error, info, warn};
 use vm_memory::{Address, Bytes, GuestAddress, GuestMemoryError, GuestMemoryMmap};
@@ -34,6 +34,9 @@ const X86_CR0_PE: u64 = 0x1;
 const X86_CR0_PG: u64 = 0x8000_0000;
 const X86_CR4_PAE: u64 = 0x20;
 
+const KBD_CMD_IO_ADDR: u16 = 0x64;
+const KBD_RESET_CMD: u8 = 0xFE;
+
 /// Errors encountered during vCPU operation.
 #[derive(Debug)]
 pub enum Error {
@@ -246,6 +249,12 @@ impl Vcpu {
                             )
                             .unwrap();
                     }
+                    KBD_CMD_IO_ADDR => {
+                        if data[0] == KBD_RESET_CMD {
+                            info!(?exit_reason, "Guest reset via keyboard controller. Bye!");
+                            process::exit(0);
+                        }
+                    }
                     _ => {
                         warn!(address = addr, "Unsupported device write at {:x?}", addr);
                     }