Add a scheduler which just keeps of processes for now, and a

with_process to interact with them. User space now has it's own address
space and mapper which means we will be able to allocate memory for it.
I added a bunch of functions as stubs into libxunil which are required
for doomgeneric.

kernel/src/task/scheduler.rs

@@ -0,0 +1,45 @@
+use alloc::collections::btree_map::BTreeMap;
+use lazy_static::lazy_static;
+
+use crate::{arch::x86_64::paging::XunilFrameAllocator, task::process::Process, util::Locked};
+
+pub struct Scheduler {
+    pub processes: BTreeMap<u64, Process>,
+    next_pid: u64,
+}
+
+impl Scheduler {
+    pub const fn new() -> Scheduler {
+        Scheduler {
+            processes: BTreeMap::new(),
+            next_pid: 1,
+        }
+    }
+}
+
+impl Locked<Scheduler> {
+    pub fn spawn_process(
+        &self,
+        entry_point: u64,
+        frame_allocator: &mut XunilFrameAllocator,
+    ) -> Option<u64> {
+        let mut guard = self.lock();
+        let pid = guard.next_pid;
+        guard.next_pid += 1;
+        let process = Process::new(pid, entry_point, frame_allocator)?;
+        guard.processes.insert(pid, process);
+
+        Some(pid)
+    }
+
+    pub fn with_process<F, R>(&self, index: u64, f: F) -> Option<R>
+    where
+        F: FnOnce(&mut Process) -> R,
+    {
+        let mut guard = self.lock();
+        let process = guard.processes.get_mut(&index)?;
+        Some(f(process))
+    }
+}
+
+pub static SCHEDULER: Locked<Scheduler> = Locked::new(Scheduler::new());