layout, main: Set identity mapping back to 4 GiB

It turns out we don't need MMIO devices above 4 GiB.

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

layout.ld

@@ -32,11 +32,11 @@ SECTIONS
 
 	firmware_ram_size = . - ram_min;
 
-	/* Memory for 64 GiB identity mapping, keep synced with ADDRESS_SPACE_GIB */
-	identity_mapped_gbs = 64;
+	/* Memory for identity mapping, keep synced with ADDRESS_SPACE_GIB */
+	address_space_gib = 4;
 	. = ALIGN(4K);
 	pml2t = .;
-	. += 4K * identity_mapped_gbs;
+	. += 4K * address_space_gib;
 
 	ASSERT((. <= ram_max - stack_size), "firmware size too big for RAM region")
 

src/main.rs

@@ -52,12 +52,12 @@ fn panic(info: &PanicInfo) -> ! {
 
 /// Setup page tables to provide an identity mapping over the full 4GiB range
 fn setup_pagetables() {
-    const ADDRESS_SPACE_GIB: u64 = 64;
-    type Page = [u64; 512];
+    const ADDRESS_SPACE_GIB: u64 = 4;
+    type PageTable = [u64; 512];
 
     extern "C" {
-        static pml3t: Page;
-        static pml2t: [Page; ADDRESS_SPACE_GIB as usize];
+        static pml3t: PageTable;
+        static pml2t: [PageTable; ADDRESS_SPACE_GIB as usize];
     }
 
     let pte = mem::MemoryRegion::from_slice(unsafe { &pml2t });