uservec before userret in trampoline, to match book and kernelvec

kernel/kernelvec.S

@@ -8,8 +8,10 @@
 .globl kernelvec
 .align 4
 kernelvec:
+        // make room to save registers.
         addi sp, sp, -256
 
+        // save the registers.
         sd ra, 0(sp)
         sd sp, 8(sp)
         sd gp, 16(sp)
@@ -42,8 +44,10 @@ kernelvec:
         sd t5, 232(sp)
         sd t6, 240(sp)
 
+	// call the C trap handler in trap.c
         call kerneltrap
 
+        // restore registers.
         ld ra, 0(sp)
         ld sp, 8(sp)
         ld gp, 16(sp)
@@ -78,6 +82,7 @@ kernelvec:
 
         addi sp, sp, 256
 
+        // return to whatever we were doing in the kernel.
         sret
 
         #

kernel/trampoline.S

@@ -2,77 +2,21 @@
         # code to switch between user and kernel space.
         #
         # this code is mapped at the same virtual address
-        # in user and kernel space so that it continues
+        # (TRAMPOLINE) in user and kernel space so that
-        # to work when it switches page tables.
+        # it continues to work when it switches page tables.
 	#
-	# kernel.ld causes userret to be aligned
+	# kernel.ld causes this to be aligned
         # to a page boundary.
         #
 	.section trampsec
 .globl trampoline
 trampoline:
-.globl userret
-userret:
-        # userret(trapframe, pagetable)
-        # switch from kernel to user.
-        # usertrapret() calls here.
-	# a0: p->tf in user page table
-        # a1: new value for satp, for user page table
-
-	# switch to the user page table.
-	sfence.vma zero, zero
-        csrw satp, a1
-
-        # put the saved user a0 in sscratch, so we
-        # can swap it with our a0 (p->tf) in the last step.
-        ld t0, 112(a0)
-        csrw sscratch, t0
-
-        # restore all but a0 from p->tf
-        ld ra, 40(a0)
-        ld sp, 48(a0)
-        ld gp, 56(a0)
-        ld tp, 64(a0)
-        ld t0, 72(a0)
-        ld t1, 80(a0)
-        ld t2, 88(a0)
-        ld s0, 96(a0)
-        ld s1, 104(a0)
-        ld a1, 120(a0)
-        ld a2, 128(a0)
-        ld a3, 136(a0)
-        ld a4, 144(a0)
-        ld a5, 152(a0)
-        ld a6, 160(a0)
-        ld a7, 168(a0)
-        ld s2, 176(a0)
-        ld s3, 184(a0)
-        ld s4, 192(a0)
-        ld s5, 200(a0)
-        ld s6, 208(a0)
-        ld s7, 216(a0)
-        ld s8, 224(a0)
-        ld s9, 232(a0)
-        ld s10, 240(a0)
-        ld s11, 248(a0)
-        ld t3, 256(a0)
-        ld t4, 264(a0)
-        ld t5, 272(a0)
-        ld t6, 280(a0)
-
-	# restore user a0, and save p->tf in sscratch
-        csrrw a0, sscratch, a0
-        
-        # return to user mode and user pc.
-        # usertrapret() set up sstatus and sepc.
-        sret
-
 .align 4
 .globl uservec
 uservec:    
 	#
-        # trap.c set stvec to point here, so
+        # trap.c sets stvec to point here, so
-        # user interrupts and exceptions start here,
+        # traps from user space start here,
         # in supervisor mode, but with a
         # user page table.
         #
@@ -139,3 +83,59 @@ uservec:
 
         # jump to usertrap(), which does not return
         jr t0
+
+.globl userret
+userret:
+        # userret(trapframe, pagetable)
+        # switch from kernel to user.
+        # usertrapret() calls here.
+	# a0: p->tf in user page table
+        # a1: new value for satp, for user page table
+
+	# switch to the user page table.
+	sfence.vma zero, zero
+        csrw satp, a1
+
+        # put the saved user a0 in sscratch, so we
+        # can swap it with our a0 (p->tf) in the last step.
+        ld t0, 112(a0)
+        csrw sscratch, t0
+
+        # restore all but a0 from p->tf
+        ld ra, 40(a0)
+        ld sp, 48(a0)
+        ld gp, 56(a0)
+        ld tp, 64(a0)
+        ld t0, 72(a0)
+        ld t1, 80(a0)
+        ld t2, 88(a0)
+        ld s0, 96(a0)
+        ld s1, 104(a0)
+        ld a1, 120(a0)
+        ld a2, 128(a0)
+        ld a3, 136(a0)
+        ld a4, 144(a0)
+        ld a5, 152(a0)
+        ld a6, 160(a0)
+        ld a7, 168(a0)
+        ld s2, 176(a0)
+        ld s3, 184(a0)
+        ld s4, 192(a0)
+        ld s5, 200(a0)
+        ld s6, 208(a0)
+        ld s7, 216(a0)
+        ld s8, 224(a0)
+        ld s9, 232(a0)
+        ld s10, 240(a0)
+        ld s11, 248(a0)
+        ld t3, 256(a0)
+        ld t4, 264(a0)
+        ld t5, 272(a0)
+        ld t6, 280(a0)
+
+	# restore user a0, and save p->tf in sscratch
+        csrrw a0, sscratch, a0
+        
+        # return to user mode and user pc.
+        # usertrapret() set up sstatus and sepc.
+        sret

kernel/trap.c

@@ -9,7 +9,7 @@
 struct spinlock tickslock;
 uint ticks;
 
-extern char trampoline[], uservec[];
+extern char trampoline[], uservec[], userret[];
 
 // in kernelvec.S, calls kerneltrap().
 void kernelvec();
@@ -123,7 +123,8 @@ usertrapret(void)
   // jump to trampoline.S at the top of memory, which 
   // switches to the user page table, restores user registers,
   // and switches to user mode with sret.
-  ((void (*)(uint64,uint64))TRAMPOLINE)(TRAPFRAME, satp);
+  uint64 fn = TRAMPOLINE + (userret - trampoline);
+  ((void (*)(uint64,uint64))fn)(TRAPFRAME, satp);
 }
 
 // interrupts and exceptions from kernel code go here via kernelvec,