uefi: Add global system table pointer and API

This is a minimal solution to providing a global system table so that users of
the API don't have to pass around references to `SystemTable` / `BootServices` /
`RuntimeServices` everywhere.

Author: nicholasbishop

uefi/CHANGELOG.md

@@ -1,5 +1,10 @@
 # uefi - [Unreleased]
 
+## Added
+- Added `table::{set_system_table, system_table_boot, system_table_runtime}`.
+  This provides an initial API for global tables that do not require passing
+  around a reference.
+
 ## Removed
 - Removed the `panic-on-logger-errors` feature of the `uefi` crate. Logger
   errors are now silently ignored.

uefi/src/table/mod.rs

@@ -11,6 +11,68 @@ pub use header::Header;
 pub use system::{Boot, Runtime, SystemTable};
 pub use uefi_raw::table::Revision;
 
+use core::ptr;
+use core::sync::atomic::{AtomicPtr, Ordering};
+
+/// Global system table pointer. This is only modified by [`set_system_table`].
+static SYSTEM_TABLE: AtomicPtr<uefi_raw::table::system::SystemTable> =
+    AtomicPtr::new(ptr::null_mut());
+
+/// Update the global system table pointer.
+///
+/// This is called automatically in the `main` entry point as part of
+/// [`uefi::entry`]. It should not be called at any other point in time, unless
+/// the executable does not use [`uefi::entry`], in which case it should be
+/// called once before calling any other API in this crate.
+///
+/// # Safety
+///
+/// This function should only be called as described above, and the
+/// `ptr` must be a valid image [`SystemTable`].
+pub unsafe fn set_system_table(ptr: *const uefi_raw::table::system::SystemTable) {
+    SYSTEM_TABLE.store(ptr.cast_mut(), Ordering::Release);
+}
+
+/// Get the system table while boot services are active.
+///
+/// # Panics
+///
+/// Panics if the system table has not been set with `set_system_table`, or if
+/// boot services are not active.
+pub fn system_table_boot() -> SystemTable<Boot> {
+    let st = SYSTEM_TABLE.load(Ordering::Acquire);
+    assert!(!st.is_null());
+
+    // SAFETY: the system table is valid per the requirements of `set_system_table`.
+    unsafe {
+        if (*st).boot_services.is_null() {
+            panic!("boot services are not active");
+        }
+
+        SystemTable::<Boot>::from_ptr(st.cast()).unwrap()
+    }
+}
+
+/// Get the system table while runtime services are active.
+///
+/// # Panics
+///
+/// Panics if the system table has not been set with `set_system_table`, or if
+/// runtime services are not active.
+pub fn system_table_runtime() -> SystemTable<Runtime> {
+    let st = SYSTEM_TABLE.load(Ordering::Acquire);
+    assert!(!st.is_null());
+
+    // SAFETY: the system table is valid per the requirements of `set_system_table`.
+    unsafe {
+        if (*st).runtime_services.is_null() {
+            panic!("boot services are not active");
+        }
+
+        SystemTable::<Runtime>::from_ptr(st.cast()).unwrap()
+    }
+}
+
 /// Common trait implemented by all standard UEFI tables.
 pub trait Table {
     /// A unique number assigned by the UEFI specification