x86: Introduce helpers/checks for endbr64 instructions

... to prevent the optimiser creating unsafe code.  See the code comment for
full details.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 4046ba9744)

Note: For the backport to 4.14 thru 4.16, we don't care for embedded endbr64
      specifically, but place_endbr64() is a prerequisite for other parts of
      the series.

xen/include/asm-x86/endbr.h

@@ -0,0 +1,55 @@
+/******************************************************************************
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Copyright (c) 2021-2022 Citrix Systems Ltd.
+ */
+#ifndef XEN_ASM_ENDBR_H
+#define XEN_ASM_ENDBR_H
+
+#include <xen/types.h>
+
+#define ENDBR64_LEN 4
+
+/*
+ * In some cases we need to inspect/insert endbr64 instructions.
+ *
+ * The naive way, mem{cmp,cpy}(ptr, "\xf3\x0f\x1e\xfa", 4), optimises unsafely
+ * by placing 0xfa1e0ff3 in an imm32 operand, and marks a legal indirect
+ * branch target as far as the CPU is concerned.
+ *
+ * gen_endbr64() is written deliberately to avoid the problematic operand, and
+ * marked __const__ as it is safe for the optimiser to hoist/merge/etc.
+ */
+static inline uint32_t __attribute_const__ gen_endbr64(void)
+{
+    uint32_t res;
+
+    asm ( "mov $~0xfa1e0ff3, %[res]\n\t"
+          "not %[res]\n\t"
+          : [res] "=&r" (res) );
+
+    return res;
+}
+
+static inline bool is_endbr64(const void *ptr)
+{
+    return *(const uint32_t *)ptr == gen_endbr64();
+}
+
+static inline void place_endbr64(void *ptr)
+{
+    *(uint32_t *)ptr = gen_endbr64();
+}
+
+#endif /* XEN_ASM_ENDBR_H */