Add kernel stack for exception handling

Signed-off-by: Harry Chen <i@harrychen.xyz>

.gitignore

@@ -13,3 +13,5 @@ Cargo.lock
 
 # for eclipse
 .project
+
+.vscode

kernel/src/arch/mipsel/boot/entry.S

@@ -5,9 +5,22 @@
     .globl _start
 
 _start:
-	la sp, bootstacktop
+    # setup stack and gp
-    la gp, _gp
+	la    sp, bootstacktop
+    la    gp, _gp
 
+    # set ebase
+    la    t0, trap_entry
+    mfc0  t1, 15 # C0_EBASE
+    or    t1, t1, t0
+    mtc0  t1, 15
+
+    # exit bootstrap mode
+    mfc0  t0, 12 # C0_STATUS
+    andi  t0, t0, 0xFFBFFFFF # set BEV (bit 22) to 0
+    mtc0  t0, 12
+
+    # directly jump to main function
     b rust_main
 	nop
 

kernel/src/arch/mipsel/boot/linker.ld

@@ -17,6 +17,8 @@ SECTIONS
         *(.text.entry)
         *(.text .text.*)
         . = ALIGN(4K);
+        *(.text.ebase)
+        . = ALIGN(4K);
         etext = .;
     }
 

kernel/src/arch/mipsel/boot/trap.S

@@ -2,8 +2,10 @@
 
 	.set noat
 	.set noreorder
-    .section .text
+    .section .text.ebase
     .globl trap_entry
+
+.org 0x0
 trap_entry:
 # +0x000: TLB-miss vector
 	b general_trap_vec
@@ -18,10 +20,9 @@ general_trap_vec:
 	nop  # delayslot
 
 trap_from_user:
-	# TODO: load kstack, we can use k0 to store something
+	# load kstack, we can use k0 to store something
-	# la     k0, address_of_kstack
+	la     k0, kernel_stack
-	# addiu  sp, k0, size_of_kstack
+	la     sp, kernel_stack_top
-	nop
 
 trap_from_kernel:
 	/* 
@@ -92,7 +93,7 @@ trap_from_kernel:
 
 	# prepare to call rust_trap
 	jal rust_trap
-	addiu a0, sp, 16             /* set argument */
+	addiu a0, sp, 16             /* set argument (trapframe) */
 
 	.globl trap_return
 trap_return:
@@ -148,3 +149,10 @@ trap_return:
 	eret
 	nop
 
+    .section .bss.stack
+    .align 12  #PGSHIFT
+    .global kernel_stack
+kernel_stack:
+    .space 1024 * 16 # 16KB for kernel stack
+    .global kernel_stack_top
+kernel_stack_top: