Implement HlReadGSQuadWord() intrinsic

sdk/xtdk/amd64/hlfuncs.h

@@ -67,6 +67,10 @@ XTCDECL
 ULONG_PTR
 HlReadControlRegister(IN USHORT ControlRegister);
 
+XTCDECL
+ULONGLONG
+HlReadGSQuadWord(ULONG Offset);
+
 XTCDECL
 ULONGLONG
 HlReadModelSpecificRegister(IN ULONG Register);

xtoskrnl/hl/amd64/cpufunc.c

@@ -318,6 +318,27 @@ HlReadControlRegister(IN USHORT ControlRegister)
     return Value;
 }
 
+/**
+ * Reads quadword from a memory location specified by an offset relative to the beginning of the GS segment.
+ *
+ * @param Offset
+ *        Specifies the offset from the beginning of GS segment.
+ *
+ * @return Returns the value read from the specified memory location relative to GS segment.
+ *
+ * @since XT 1.0
+ */
+XTCDECL
+ULONGLONG
+HlReadGSQuadWord(ULONG Offset)
+{
+    ULONGLONG Value;
+
+    /* Read quadword from GS segment */
+    asm volatile("movq %%gs:%a[Offset], %q[Value]" : [Value] "=r" (Value) : [Offset] "ir" (Offset));
+    return Value;
+}
+
 /**
  * Reads a 64-bit value from the requested Model Specific Register (MSR).
  *