sp1/amd64: head: Add kfence.h

Signed-off-by: Ian Moffett <ian@mirocom.org>

usr/src/sp1/head/amd64/kfence.h

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2026, Mirocom Laboratories
+ * All rights reserved.
+ *
+ * The following sources are CONFIDENTIAL and PROPRIETARY
+ * property of Mirocom Laboratories. Unauthorized copying,
+ * use, distribution or modification of this file, in whole
+ * and in part, is strictly prohibited without the prior written
+ * consent from Mirocom Laboratories.
+ */
+
+#ifndef _MACHINE_KFENCE_H_
+#define _MACHINE_KFENCE_H_ 1
+
+/*
+ * Kernel fence for interrupt entries that do
+ * not have an error code
+ */
+#define KFENCE              \
+    testq $0x3, 8(%rsp)   ; \
+    jz 1f                 ; \
+    lfence                ; \
+    swapgs                ; \
+1:  nop
+
+/*
+ * Kernel fence for interrupt entries that
+ * have an error code
+ */
+#define KFENCE_EC           \
+    testq $0x3, 16(%rsp)  ; \
+    jz 1f                 ; \
+    lfence                ; \
+    swapgs                ; \
+1:  nop
+
+#endif  /* !_MACHINE_KFENCE_H_ *