nemu: support yield()

abstract-machine/am/src/riscv/nemu/cte.c

@@ -2,7 +2,6 @@
 #include <am.h>
 #include <klib.h>
 #include <riscv/riscv.h>
-#include <stdint.h>
 
 static Context *(*user_handler)(Event, Context *) = NULL;
 
@@ -38,10 +37,7 @@ bool cte_init(Context *(*handler)(Event, Context *)) {
 }
 
 Context *kcontext(Area kstack, void (*entry)(void *), void *arg) {
-  Context *c = kstack.end - sizeof(Context);
-  c->mepc = (uintptr_t)entry;
-  c->gpr[10] = (uintptr_t)arg;
-  return c;
+  return NULL;
 }
 
 void yield() {

abstract-machine/am/src/riscv/nemu/trap.S

@@ -60,8 +60,6 @@ __am_asm_trap:
   mv a0, sp
   jal __am_irq_handle
 
-  mv sp, a0
-
   LOAD t1, OFFSET_STATUS(sp)
   LOAD t2, OFFSET_EPC(sp)
   csrw mstatus, t1

nemu/src/cpu/cpu-exec.c

@@ -158,6 +158,8 @@ breakpoint_t *cpu_exec_with_bp(uint64_t n, breakpoint_t *bp, size_t len) {
   static Decode s;
   nemu_state.state = NEMU_RUNNING;
   do {
+    exec_once(&s, cpu.pc);
+    g_nr_guest_inst++;
     for (int i = 0; i < len; i++) {
       size_t addr = bp[i].addr;
       bp_type_t bptype = bp[i].type;
@@ -174,8 +176,6 @@ breakpoint_t *cpu_exec_with_bp(uint64_t n, breakpoint_t *bp, size_t len) {
         return bp + i;
       }
     }
-    exec_once(&s, cpu.pc);
-    g_nr_guest_inst++;
     if (nemu_state.state != NEMU_RUNNING)
       return NULL;
   } while (--n);