arch-x86: Implement real mode far ret.

Change-Id: I4fd3210f30246f19ca03906465f160bcbfbfbccc
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55450
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/x86/isa/decoder/one_byte_opcodes.isa

@@ -388,7 +388,7 @@
                 0x1: Inst::LEAVE();
                 0x2: ret_far_Iw();
                 0x3: decode MODE_SUBMODE {
-                    0x3, 0x4: ret_far_real();
+                    0x3, 0x4: Inst::RET_FAR_REAL();
                     default: Inst::RET_FAR();
                 }
                 0x4: Inst::INT3();

src/arch/x86/isa/insts/general_purpose/control_transfer/xreturn.py

@@ -62,6 +62,29 @@ def macroop RET_NEAR_I
     wripi t1, 0
 };
 
+def macroop RET_FAR_REAL {
+    .function_return
+    .control_indirect
+
+    # Pop the return RIP.
+    ld t1, ss, [1, t0, rsp], addressSize=ssz
+    # Pop the return CS.
+    ld t2, ss, [1, t0, rsp], dsz, addressSize=ssz
+    # Adjust RSP.
+    addi rsp, rsp, "2 * env.dataSize", dataSize=ssz
+
+    # Set the CS selector.
+    wrsel cs, t2
+    # Make sure there isn't any junk in the upper bits of the base.
+    mov t2, t0, t2
+    # Compute and set CS base.
+    slli t2, t2, 4, dataSize=8
+    wrbase cs, t2, dataSize=8
+
+    # Set the new RIP.
+    wrip t1, t0
+};
+
 def macroop RET_FAR {
     .adjust_env oszIn64Override
     .function_return