exec: Rename exec-vary.c as page-vary.c

exec-vary.c is about variable page size handling,
rename it page-vary.c. Currently this file is target
specific (built once for each target), comment this.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210322112427.4045204-2-f4bug@amsat.org>
[rth: Update MAINTAINERS]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

page-vary.c

@@ -0,0 +1,108 @@
+/*
+ * Variable page size handling -- target specific part.
+ *
+ *  Copyright (c) 2003 Fabrice Bellard
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "qemu/osdep.h"
+#include "qemu-common.h"
+
+#define IN_EXEC_VARY 1
+
+#include "exec/exec-all.h"
+
+#ifdef TARGET_PAGE_BITS_VARY
+# ifdef CONFIG_ATTRIBUTE_ALIAS
+/*
+ * We want to declare the "target_page" variable as const, which tells
+ * the compiler that it can cache any value that it reads across calls.
+ * This avoids multiple assertions and multiple reads within any one user.
+ *
+ * This works because we finish initializing the data before we ever read
+ * from the "target_page" symbol.
+ *
+ * This also requires that we have a non-constant symbol by which we can
+ * perform the actual initialization, and which forces the data to be
+ * allocated within writable memory.  Thus "init_target_page", and we use
+ * that symbol exclusively in the two functions that initialize this value.
+ *
+ * The "target_page" symbol is created as an alias of "init_target_page".
+ */
+static TargetPageBits init_target_page;
+
+/*
+ * Note that this is *not* a redundant decl, this is the definition of
+ * the "target_page" symbol.  The syntax for this definition requires
+ * the use of the extern keyword.  This seems to be a GCC bug in
+ * either the syntax for the alias attribute or in -Wredundant-decls.
+ *
+ * See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91765
+ */
+#  pragma GCC diagnostic push
+#  pragma GCC diagnostic ignored "-Wredundant-decls"
+
+extern const TargetPageBits target_page
+    __attribute__((alias("init_target_page")));
+
+#  pragma GCC diagnostic pop
+# else
+/*
+ * When aliases are not supported then we force two different declarations,
+ * by way of suppressing the header declaration with IN_EXEC_VARY.
+ * We assume that on such an old compiler, LTO cannot be used, and so the
+ * compiler cannot not detect the mismatched declarations, and all is well.
+ */
+TargetPageBits target_page;
+#  define init_target_page target_page
+# endif
+#endif
+
+bool set_preferred_target_page_bits(int bits)
+{
+    /*
+     * The target page size is the lowest common denominator for all
+     * the CPUs in the system, so we can only make it smaller, never
+     * larger. And we can't make it smaller once we've committed to
+     * a particular size.
+     */
+#ifdef TARGET_PAGE_BITS_VARY
+    assert(bits >= TARGET_PAGE_BITS_MIN);
+    if (init_target_page.bits == 0 || init_target_page.bits > bits) {
+        if (init_target_page.decided) {
+            return false;
+        }
+        init_target_page.bits = bits;
+    }
+#endif
+    return true;
+}
+
+void finalize_target_page_bits(void)
+{
+#ifdef TARGET_PAGE_BITS_VARY
+    if (init_target_page.bits == 0) {
+        init_target_page.bits = TARGET_PAGE_BITS_MIN;
+    }
+    init_target_page.mask = (target_long)-1 << init_target_page.bits;
+    init_target_page.decided = true;
+
+    /*
+     * For the benefit of an -flto build, prevent the compiler from
+     * hoisting a read from target_page before we finish initializing.
+     */
+    barrier();
+#endif
+}