task/schedule: Launch elf tasks at CPL-3

In order to provide separation between loadable modules and
between a module and the kernel, loadable modules should be
executed at CPL-3 and use a syscall interface to interact with the
SVSM kernel.

This commit simply changes the creation of tasks from ELF files
from using kernel tasks to user-mode tasks running at CPL-3.

Signed-off-by: Roy Hopkins <rhopkins@suse.de>

src/task/schedule.rs

@@ -357,7 +357,7 @@ pub fn create_task_for_module(
     flags: u16,
     affinity: Option<u32>,
 ) -> Result<(), SvsmError> {
-    let mut task = Task::create(module.entry_point(), 0, flags)?;
+    let mut task = Task::user_create(module.entry_point(), 0, flags)?;
     task.set_affinity(affinity);
     task.set_on_switch_hook(Some(task_switch_hook));
 

src/task/tasks.rs

@@ -62,7 +62,7 @@ impl From<TaskError> for SvsmError {
 
 #[derive(Clone, Copy, Debug)]
 struct UserParams {
-    entry_point: usize,
+    entry_point: extern "C" fn(u64),
     param: u64,
 }
 
@@ -281,7 +281,11 @@ impl Task {
         Ok(task)
     }
 
-    pub fn user_create(entry: usize, param: u64, flags: u16) -> Result<Box<Task>, SvsmError> {
+    pub fn user_create(
+        entry: extern "C" fn(u64),
+        param: u64,
+        flags: u16,
+    ) -> Result<Box<Task>, SvsmError> {
         // Launch via the user-mode entry point
         let entry_param = Box::new(UserParams {
             entry_point: entry,