Read maximum paging depth directly from CR4 for x86_64 constants

.github/workflows/post-commit-workflow.yaml

@@ -4,7 +4,7 @@ on:
   - pull_request
 
 env:
-  NIGHTLY_VERSION: nightly-2025-07-25
+  NIGHTLY_VERSION: nightly-2025-08-10
 
 defaults:
   run:

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "libsys"
-version = "0.3.3"
+version = "0.3.4"
 edition = "2024"
 
 [dependencies]

rust-toolchain.toml

@@ -1,4 +1,4 @@
 [toolchain]
-channel = "nightly-2025-07-25"
+channel = "nightly-2025-08-10"
 components = ["rust-src", "rustfmt", "clippy"]
 profile = "minimal"

src/constants/mod.rs

@@ -1,38 +1,4 @@
-#![allow(clippy::missing_panics_doc)]
-
-use core::num::NonZero;
-
 #[cfg(target_arch = "x86_64")]
 mod x86_64;
 #[cfg(target_arch = "x86_64")]
 pub use x86_64::*;
-
-#[cfg(not(test))]
-static PAGING_DEPTH: spin::Once<NonZero<u32>> = spin::Once::new();
-
-/// Sets the current paging depth.
-///
-/// # Safety
-///
-/// - `value` must be the current paging depth.
-///
-/// # Remarks
-///
-/// If `value` is *not* the current paging depth, it will be possible to
-/// construct invalid [`Address`][crate::address::Address] kinds that rely on
-/// checking the current paging depth to ensure canonicality.
-#[cfg(not(test))]
-pub unsafe fn set_paging_depth(paging_depth: NonZero<u32>) {
-    PAGING_DEPTH.call_once(|| paging_depth);
-}
-
-pub fn get_paging_depth() -> NonZero<u32> {
-    cfg_select! {
-        test => {
-            // Safety: Value is non-zero.
-            unsafe { NonZero::new_unchecked(4) }
-        }
-
-        _ => { *PAGING_DEPTH.get().expect("paging depth has not been set") }
-    }
-}

src/constants/x86_64.rs

@@ -1,6 +1,36 @@
-use crate::constants::get_paging_depth;
 use core::num::NonZero;
 
+#[cfg(test)]
+#[must_use]
+pub fn get_paging_depth() -> NonZero<u32> {
+    // Safety: Value is non-zero.
+    unsafe { NonZero::new_unchecked(4) }
+}
+
+#[cfg(not(test))]
+#[must_use]
+pub fn get_paging_depth() -> NonZero<u32> {
+    const CR4_LA57_BIT: usize = 1 << 12;
+
+    let cr4: usize;
+
+    unsafe {
+        core::arch::asm!(
+            "mov {}, cr4",
+            out(reg) cr4,
+            options(nostack, nomem, preserves_flags)
+        );
+    }
+
+    if (cr4 & CR4_LA57_BIT) == 0 {
+        // Safety: Value is non-zero.
+        unsafe { NonZero::new_unchecked(4) }
+    } else {
+        // Safety: Value is non-zero.
+        unsafe { NonZero::new_unchecked(5) }
+    }
+}
+
 /// Bit shift required to offset page indexes.
 #[must_use]
 pub const fn page_bits() -> NonZero<u32> {
@@ -105,17 +135,17 @@ pub const fn is_physical_address_canonical(physical_address: usize) -> bool {
 /// Bit-shift to reach non-canonical bits of a virtual address.
 #[must_use]
 pub fn virtual_address_bits() -> NonZero<u32> {
-    let table_indexes_shift = table_index_bits().checked_mul(get_paging_depth()).unwrap();
-    let total_shift = table_indexes_shift.checked_add(page_bits().get()).unwrap();
+    let table_indexes_shift = table_index_bits().get() * get_paging_depth().get();
+    let total_shift = table_indexes_shift + page_bits().get();
 
-    NonZero::<u32>::new(total_shift.get()).unwrap()
+    // Safety: Operations cannot overflow with the provided constants.
+    unsafe { NonZero::<u32>::new_unchecked(total_shift) }
 }
 
 /// Checks whether a provided address has only the canonical virtual bits.
 #[must_use]
 pub fn is_virtual_address_canonical(virtual_address: usize) -> bool {
-    let sign_extension_check_shift = virtual_address_bits().get().checked_sub(1).unwrap();
-
+    let sign_extension_check_shift = virtual_address_bits().get() - 1;
     matches!(virtual_address >> sign_extension_check_shift, 0 | 0x1FFFF)
 }