add ringswitch stuff

kernel/arch/core_ring.cc

@@ -11,6 +11,30 @@ void switchToUsermode(void *stackpointer, void *kickoff,
 	(void)stackpointer;
 	(void)kickoff;
 	(void)kickoff_parameter;
+
+    // Segment Selectors for Ring 3
+    constexpr uint16_t user_cs = 3 << 3 | 3;
+    constexpr uint16_t user_ds = 4 << 3 | 3;
+
+    asm volatile ( \
+            "pushq %[user_data] \n" \
+            "pushq %[user_stack] \n" \
+            "pushq %[user_eflags] \n" \
+            "pushq %[user_code] \n" \
+            "pushq %[user_function] \n" \
+            "mov %[user_parameter], %%rdi\n" \
+            "iretq \n" \
+            : \
+            : [user_data] "r"((uint64_t)user_ds), 
+              [user_code] "r"((uint64_t)user_cs), 
+              [user_stack] "r"((uint64_t)stackpointer), 
+              [user_eflags] "r"((uint64_t)1<<9 | 2),  // Interrupt Enable Flag
+              [user_function] "r"((uint64_t)kickoff), 
+              [user_parameter] "r"((uint64_t)kickoff_parameter)
+            \
+            : "memory", "rdi"\
+            );
+
 	__builtin_unreachable();
 }
 }  // namespace Ring