From 8b380f0692c96b7f0d2c5c1335b26574d71d3626 Mon Sep 17 00:00:00 2001
From: Philipp Oppermann <dev@phil-opp.com>
Date: Sat, 26 Jan 2019 12:52:47 +0100
Subject: [PATCH] Create a new memory module with a `translate_addr` function

---
 src/lib.rs    |  1 +
 src/memory.rs | 43 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)
 create mode 100644 src/memory.rs

diff --git a/src/lib.rs b/src/lib.rs
index 34debc4f..5974e73c 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -3,6 +3,7 @@
 
 pub mod gdt;
 pub mod interrupts;
+pub mod memory;
 pub mod serial;
 pub mod vga_buffer;
 
diff --git a/src/memory.rs b/src/memory.rs
new file mode 100644
index 00000000..76837ef7
--- /dev/null
+++ b/src/memory.rs
@@ -0,0 +1,43 @@
+use x86_64::structures::paging::PageTable;
+use x86_64::PhysAddr;
+
+/// Returns the physical address for the given virtual address, or `None` if the
+/// virtual address is not mapped.
+pub fn translate_addr(addr: usize, level_4_table_addr: usize) -> Option<PhysAddr> {
+    // retrieve the page table indices of the address that we want to translate
+    let level_4_index = (addr >> 39) & 0o777;
+    let level_3_index = (addr >> 30) & 0o777;
+    let level_2_index = (addr >> 21) & 0o777;
+    let level_1_index = (addr >> 12) & 0o777;
+    let page_offset = addr & 0o7777;
+
+    // check that level 4 entry is mapped
+    let level_4_table = unsafe { &*(level_4_table_addr as *const PageTable) };
+    if level_4_table[level_4_index].addr().is_null() {
+        return None;
+    }
+    let level_3_table_addr = (level_4_table_addr << 9) | (level_4_index << 12);
+
+    // check that level 3 entry is mapped
+    let level_3_table = unsafe { &*(level_3_table_addr as *const PageTable) };
+    if level_3_table[level_3_index].addr().is_null() {
+        return None;
+    }
+    let level_2_table_addr = (level_3_table_addr << 9) | (level_3_index << 12);
+
+    // check that level 2 entry is mapped
+    let level_2_table = unsafe { &*(level_2_table_addr as *const PageTable) };
+    if level_2_table[level_2_index].addr().is_null() {
+        return None;
+    }
+    let level_1_table_addr = (level_2_table_addr << 9) | (level_2_index << 12);
+
+    // check that level 1 entry is mapped and retrieve physical address from it
+    let level_1_table = unsafe { &*(level_1_table_addr as *const PageTable) };
+    let phys_addr = level_1_table[level_1_index].addr();
+    if phys_addr.is_null() {
+        return None;
+    }
+
+    Some(phys_addr + page_offset)
+}