Add some TODOs in IOMMU initialization

ostd/src/arch/x86/iommu/dma_remapping/mod.rs

@@ -74,6 +74,11 @@ pub fn init() {
     // Create a Root Table instance.
     let mut root_table = RootTable::new();
     // For all PCI devices, use the same page table.
+    //
+    // TODO: The BIOS reserves some memory regions as DMA targets and lists them in the Reserved
+    // Memory Region Reporting (RMRR) structures. These regions must be mapped for the hardware or
+    // firmware to function properly. For more details, see Intel(R) Virtualization Technology for
+    // Directed I/O (Revision 5.0), 3.16 Handling Requests to Reserved System Memory.
     let page_table = PageTable::<DeviceMode, PageTableEntry, PagingConsts>::empty();
     for table in PciDeviceLocation::all() {
         root_table.specify_device_page_table(table, unsafe { page_table.shallow_copy() })

ostd/src/arch/x86/iommu/registers/mod.rs

@@ -258,6 +258,15 @@ impl IommuRegisters {
 
         let base_address = dmar
             .remapping_iter()
+            // TODO: Add support for multiple DMA remapping hardware unit definitions (DRHDs). Note
+            // that we use `rev()` here to select the last one, since DRHDs that control specific
+            // devices tend to be reported first.
+            //
+            // For example, Intel(R) Virtualization Technology for Directed I/O (Revision 5.0), 8.4
+            // DMA Remapping Hardware Unit Definition Structure says "If a DRHD structure with
+            // INCLUDE_PCI_ALL flag Set is reported for a Segment, it must be enumerated by BIOS
+            // after all other DRHD structures for the same Segment".
+            .rev()
             .find_map(|remapping| match remapping {
                 Remapping::Drhd(drhd) => Some(drhd.register_base_addr()),
                 _ => None,