Write page about aarch64-paging.

src/SUMMARY.md

@@ -269,7 +269,7 @@
   - [Other projects](bare-metal/aps/other-projects.md)
 - [Useful crates](bare-metal/useful-crates.md)
   - [zerocopy](bare-metal/useful-crates/zerocopy.md)
-  - [aarch64_paging]()
+  - [aarch64-paging](bare-metal/useful-crates/aarch64-paging.md)
   - [buddy_system_allocator]()
   - [tinyvec]()
   - [spin and once_cell]()

src/bare-metal/useful-crates/aarch64-paging.md

@@ -0,0 +1,35 @@
+# `aarch64-paging`
+
+The [`aarch64-paging`][1] crate lets you create page tables according to the AArch64 Virtual Memory
+System Architecture.
+
+```rust,editable,compile_fail
+use aarch64_paging::{
+    idmap::IdMap,
+    paging::{Attributes, MemoryRegion},
+};
+
+const ASID: usize = 1;
+const ROOT_LEVEL: usize = 1;
+
+// Create a new page table with identity mapping.
+let mut idmap = IdMap::new(ASID, ROOT_LEVEL);
+// Map a 2 MiB region of memory as read-only.
+idmap.map_range(
+    &MemoryRegion::new(0x80200000, 0x80400000),
+    Attributes::NORMAL | Attributes::NON_GLOBAL | Attributes::READ_ONLY,
+).unwrap();
+// Set `TTBR0_EL1` to activate the page table.
+idmap.activate();
+```
+
+<details>
+
+* For now it only supports EL1, but support for other exception levels should be straightforward to
+  add.
+* This is used in Android for the [Protected VM Firmware][2].
+
+</details>
+
+[1]: https://crates.io/crates/aarch64-paging
+[2]: https://cs.android.com/android/platform/superproject/+/master:packages/modules/Virtualization/pvmfw/