kern/amd64: cpu: Add kernel fence for trap entry

A kernel fence ensures that the GS base will contain kernel data on
entry.

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

1 files changed, 51 insertions, 0 deletions

diff --git a/sys/inc/arch/amd64/kfence.h b/sys/inc/arch/amd64/kfence.h
new file mode 100644
index 0000000..110c9a8
--- /dev/null
+++ b/sys/inc/arch/amd64/kfence.h
@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2025 Ian Marco Moffett and the Osmora Team.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of Hyra nor the names of its
+ *    contributors may be used to endorse or promote products derived from
+ *    this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef _MACHINE_KENTRY_H_
+#define _MACHINE_KENTRY_H_
+
+/*
+ * Kernel entry/exit handling for traps with error codes
+ * on the stack
+ */
+#define KFENCE_EC                   \
+        testq $0x3, 16(%rsp)      ; \
+        jz 1f                     ; \
+        lfence                    ; \
+        swapgs                    ; \
+    1:
+
+#define KFENCE                    ; \
+        testq $0x3, 8(%rsp)       ; \
+        jz 1f                     ; \
+        lfence                    ; \
+        swapgs                    ; \
+    1:
+
+#endif  /* !_MACHINE_KENTRY_H_ */