layout: Explictly alocate memory regions

The pml2 tables and stack are now explictly allocated about 1 MiB.

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

Author: josephlr

layout.ld

@@ -7,19 +7,35 @@ PHDRS
   text   PT_LOAD               ;
 }
 
+/* Loaders like to put stuff in low memory (< 1M), so we don't use it. */
+ram_min = 1M;
+ram_max = 2M;
+/* Our stack grows down from ram_max. TODO: Add a guard for stack overflows. */
+stack_size = 64K;
+
+/* Pagetable locations loaded by Firecracker/cloud-hypervisor */
+pml4t = 0x9000;
+pml3t = 0xa000;
+
 SECTIONS
 {
-	. = 1M ;
-	_start_of_file = . ;
-
 	/* Mapping in the program headers makes it easier to mmap the whole file. */
-	. += SIZEOF_HEADERS ;
+	. = ram_min;
+	. += SIZEOF_HEADERS;
 
 	.rodata : { *(.rodata .rodata.*)            } :rodata
 	.data   : { *(.data .data.*) *(.bss .bss.*) } :data
 	.text   : { *(.text .text.*)                } :text
 
-	_end_of_file = . ;
+	firmware_ram_size = . - ram_min;
+
+	/* Memory for 64 GiB identity mapping, keep synced with ADDRESS_SPACE_GIB */
+	identity_mapped_gbs = 64;
+	. = ALIGN(4K);
+	pml2t = .;
+	. += 4K * identity_mapped_gbs;
+
+	ASSERT((. <= ram_max - stack_size), "firmware size too big for RAM region")
 
 	/* Match edk2's GccBase.lds DISCARD section */
 	/DISCARD/ : {

src/main.rs

@@ -65,14 +65,22 @@ fn enable_sse2() {
 /// Setup page tables to provide an identity mapping over the full 4GiB range
 fn setup_pagetables() {
     const ADDRESS_SPACE_GIB: u64 = 64;
-    let pte = mem::MemoryRegion::new(0xb000, 512 * ADDRESS_SPACE_GIB * 8);
+    type Page = [u64; 512];
+
+    extern "C" {
+        static pml3t: Page;
+        static pml2t: [Page; ADDRESS_SPACE_GIB as usize];
+    }
+
+    let pte = mem::MemoryRegion::from_slice(unsafe { &pml2t });
     for i in 0..(512 * ADDRESS_SPACE_GIB) {
         pte.io_write_u64(i * 8, (i << 21) + 0x83u64)
     }
 
-    let pde = mem::MemoryRegion::new(0xa000, 4096);
+    let pml2t_addr = unsafe { pml2t.as_ptr() } as usize as u64;
+    let pde = mem::MemoryRegion::from_slice(unsafe { &pml3t });
     for i in 0..ADDRESS_SPACE_GIB {
-        pde.io_write_u64(i * 8, (0xb000u64 + (0x1000u64 * i)) | 0x03);
+        pde.io_write_u64(i * 8, (pml2t_addr + (0x1000u64 * i)) | 0x03);
     }
 
     log!("Page tables setup");