X86: Implement the swapgs instruction.

src/arch/x86/isa/decoder/two_byte_opcodes.isa

@@ -128,7 +128,7 @@
                         0x4: smsw_Rv();
                         0x6: lmsw_Rv();
                         0x7: decode MODRM_RM {
-                            0x0: swapgs();
+                            0x0: Inst::SWAPGS();
                             0x1: rdtscp();
                             default: Inst::UD2();
                         }

src/arch/x86/isa/insts/system/segmentation.py

@@ -167,4 +167,12 @@ def macroop LIDT_16_P
     wrbase idtr, t2
     wrlimit idtr, t1
 };
+
+def macroop SWAPGS
+{
+    rdval t1, kernel_gs_base, dataSize=8
+    rdbase t2, gs, dataSize=8
+    wrbase gs, t1, dataSize=8
+    wrval kernel_gs_base, t2, dataSize=8
+};
 '''