[src/trunk]: src/sys/external/bsd/drm2/include/linux array_index_nospec

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/e25436207d9f
branches:  trunk
changeset: 1028048:e25436207d9f
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Sun Dec 19 01:44:02 2021 +0000

description:
array_index_nospec

diffstat:

 sys/external/bsd/drm2/include/linux/nospec.h |  70 ++++++++++++++++++++++++++++
 1 files changed, 70 insertions(+), 0 deletions(-)

diffs (74 lines):

diff -r 9474d36789a3 -r e25436207d9f sys/external/bsd/drm2/include/linux/nospec.h
--- a/sys/external/bsd/drm2/include/linux/nospec.h      Sun Dec 19 01:43:50 2021 +0000
+++ b/sys/external/bsd/drm2/include/linux/nospec.h      Sun Dec 19 01:44:02 2021 +0000
@@ -0,0 +1,70 @@
+/*     $NetBSD: nospec.h,v 1.2 2021/12/19 01:44:02 riastradh Exp $     */
+
+/*-
+ * Copyright (c) 2018 The NetBSD Foundation, Inc.
+ * All rights reserved.
+ *
+ * This code is derived from software contributed to The NetBSD Foundation
+ * by Taylor R. Campbell.
+ *
+ * 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.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. 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 FOUNDATION 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        _LINUX_NOSPEC_H_
+#define        _LINUX_NOSPEC_H_
+
+#include <sys/param.h>
+#include <sys/types.h>
+
+/*
+ * array_index_nospec(i, n)
+ *
+ *     If i < n, return i; otherwise return 0.  Guarantees that
+ *     speculative execution will behave the same even if the compiler
+ *     can prove in the caller that i < n.
+ */
+static inline size_t
+array_index_nospec(size_t i, size_t n)
+{
+       size_t j, diff, mask;
+
+       /*
+        * Let j = i, but force the compiler to assume nothing about
+        * the value of j by issuing an asm block that is tagged as
+        * reading from and writing to j but actually does nothing.
+        */
+       j = i;
+       asm volatile ("" : "=r"(j) : "r"(j));
+
+       /* If i >= n, then diff has the high bit set; otherwise clear.  */
+       diff = n - 1 - j;
+
+       /*
+        * If i >= n so that diff's high bit is set, mask = 0;
+        * otherwise mask = -1 = ~0.
+        */
+       mask = (diff >> (NBBY*sizeof(diff) - 1)) - 1;
+
+       return i & mask;
+}
+
+#endif /* _LINUX_NOSPEC_H_ */