finish implementation of running multiple processes concurrently (unsafely)

kernel.c

@@ -43,7 +43,9 @@ __attribute__((naked))
 __attribute__((aligned(4)))
 void kernel_entry(void) {
     __asm__ __volatile__(
-        "csrw sscratch, sp\n"
+        // Retrieve the kernel stack of the running process from sscratch
+        "csrrw sp, sscratch, sp\n"
+
         "addi sp, sp, -4 * 31\n"
         "sw ra, 4 * 0(sp)\n"
         "sw gp, 4 * 1(sp)\n"
@@ -76,9 +78,11 @@ void kernel_entry(void) {
         "sw s10, 4 * 28(sp)\n"
         "sw s11, 4 * 29(sp)\n"
 
+        // Retrieve and save the sp at the time of exception
         "csrr a0, sscratch\n"
         "sw a0, 4 * 30(sp)\n"
 
+        // Reset the kernel stack
         "mv a0, sp\n"
         "call handle_trap\n"
 
@@ -230,6 +234,12 @@ void yield(void) {
     if (next == current_proc)
         return;
 
+    __asm__ __volatile__(
+        "csrw sscratch, %[sscratch]\n"
+        :
+        : [sscratch] "r" ((uint32_t) &next->stack[sizeof(next->stack)])
+    );
+
     // Context switch
     struct process *prev = current_proc;
     current_proc = next;