Enable CBOs (but upgrade inval to flush for safety)

machine/minit.c

@@ -246,7 +246,7 @@ void enter_supervisor_mode(void (*fn)(uintptr_t), uintptr_t arg0, uintptr_t arg1
   mstatus = INSERT_FIELD(mstatus, MSTATUS_MPIE, 0);
   write_csr(mstatus, mstatus);
   write_csr(mscratch, MACHINE_STACK_TOP() - MENTRY_FRAME_SIZE);
-  write_csr(menvcfg, MENVCFG_SSE);
+  write_csr(menvcfg, MENVCFG_SSE | MENVCFG_CBCFE | INSERT_FIELD(0, MENVCFG_CBIE, 1));
 #ifndef __riscv_flen
   uintptr_t *p_fcsr = (uintptr_t*)(MACHINE_STACK_TOP() - MENTRY_FRAME_SIZE); // the x0's save slot
   *p_fcsr = 0;

pk/pk.c

@@ -122,6 +122,8 @@ static void run_loaded_program(size_t argc, char** argv, uintptr_t kstack_top)
     write_csr(ssp, shadow_stack_top);
   }
 
+  set_csr(senvcfg, SENVCFG_CBCFE | INSERT_FIELD(0, SENVCFG_CBIE, 1));
+
   // copy phdrs to user stack
   size_t stack_top = current.stack_top - current.phdr_size;
   memcpy_to_user((void*)stack_top, (void*)current.phdr, current.phdr_size);