Merge remote-tracking branch 'remotes/rth/tags/pull-tcg-20210323' into staging

Workaround for macos mprotect
Workaround for target_page vs -flto

# gpg: Signature made Wed 24 Mar 2021 01:40:12 GMT
# gpg:                using RSA key 7A481E78868B4DB6A85A05C064DF38E8AF7E215F
# gpg:                issuer "richard.henderson@linaro.org"
# gpg: Good signature from "Richard Henderson <richard.henderson@linaro.org>" [full]
# Primary key fingerprint: 7A48 1E78 868B 4DB6 A85A  05C0 64DF 38E8 AF7E 215F

* remotes/rth/tags/pull-tcg-20210323:
  exec: Build page-vary-common.c with -fno-lto
  exec: Extract 'page-vary.h' header
  exec: Rename exec-vary.c as page-vary.c
  tcg: Workaround macOS 11.2 mprotect bug
  tcg: Do not set guard pages on the rx portion of code_gen_buffer

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

9 files changed, 167 insertions, 149 deletions

diff --git a/MAINTAINERS b/MAINTAINERS
index 9147e9a429..10ed6d7624 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -117,6 +117,8 @@ R: Paolo Bonzini <pbonzini@redhat.com>
 S: Maintained
 F: softmmu/cpus.c
 F: cpus-common.c
+F: page-vary.c
+F: page-vary-common.c
 F: accel/tcg/
 F: accel/stubs/tcg-stub.c
 F: util/cacheinfo.c
diff --git a/configure b/configure
index 61872096a8..edf9dc8985 100755
--- a/configure
+++ b/configure
@@ -4890,21 +4890,6 @@ if  test "$plugins" = "yes" &&
 fi
 
 ########################################
-# See if __attribute__((alias)) is supported.
-# This false for Xcode 9, but has been remedied for Xcode 10.
-# Unfortunately, travis uses Xcode 9 by default.
-
-attralias=no
-cat > $TMPC << EOF
-int x = 1;
-extern const int y __attribute__((alias("x")));
-int main(void) { return 0; }
-EOF
-if compile_prog "" "" ; then
-    attralias=yes
-fi
-
-########################################
 # check if getauxval is available.
 
 getauxval=no
@@ -5935,10 +5920,6 @@ if test "$atomic64" = "yes" ; then
   echo "CONFIG_ATOMIC64=y" >> $config_host_mak
 fi
 
-if test "$attralias" = "yes" ; then
-  echo "CONFIG_ATTRIBUTE_ALIAS=y" >> $config_host_mak
-fi
-
 if test "$getauxval" = "yes" ; then
   echo "CONFIG_GETAUXVAL=y" >> $config_host_mak
 fi
diff --git a/exec-vary.c b/exec-vary.c
deleted file mode 100644
index a603b1b433..0000000000
--- a/exec-vary.c
+++ /dev/null
@@ -1,108 +0,0 @@
-/*
- * Variable page size handling
- *
- *  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
-}
diff --git a/include/exec/cpu-all.h b/include/exec/cpu-all.h
index 76443eb11d..d76b0b9e02 100644
--- a/include/exec/cpu-all.h
+++ b/include/exec/cpu-all.h
@@ -215,22 +215,15 @@ static inline void stl_phys_notdirty(AddressSpace *as, hwaddr addr, uint32_t val
 /* page related stuff */
 
 #ifdef TARGET_PAGE_BITS_VARY
-typedef struct {
-    bool decided;
-    int bits;
-    target_long mask;
-} TargetPageBits;
-#if defined(CONFIG_ATTRIBUTE_ALIAS) || !defined(IN_EXEC_VARY)
+# include "exec/page-vary.h"
 extern const TargetPageBits target_page;
-#else
-extern TargetPageBits target_page;
-#endif
 #ifdef CONFIG_DEBUG_TCG
 #define TARGET_PAGE_BITS   ({ assert(target_page.decided); target_page.bits; })
-#define TARGET_PAGE_MASK   ({ assert(target_page.decided); target_page.mask; })
+#define TARGET_PAGE_MASK   ({ assert(target_page.decided); \
+                              (target_long)target_page.mask; })
 #else
 #define TARGET_PAGE_BITS   target_page.bits
-#define TARGET_PAGE_MASK   target_page.mask
+#define TARGET_PAGE_MASK   ((target_long)target_page.mask)
 #endif
 #define TARGET_PAGE_SIZE   (-(int)TARGET_PAGE_MASK)
 #else
diff --git a/include/exec/page-vary.h b/include/exec/page-vary.h
new file mode 100644
index 0000000000..c22a7a742e
--- /dev/null
+++ b/include/exec/page-vary.h
@@ -0,0 +1,34 @@
+/*
+ * Definitions for cpus with variable page sizes.
+ *
+ *  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/>.
+ */
+
+#ifndef EXEC_PAGE_VARY_H
+#define EXEC_PAGE_VARY_H
+
+typedef struct {
+    bool decided;
+    int bits;
+    uint64_t mask;
+} TargetPageBits;
+
+#ifdef IN_PAGE_VARY
+extern bool set_preferred_target_page_bits_common(int bits);
+extern void finalize_target_page_bits_common(int min);
+#endif
+
+#endif /* EXEC_PAGE_VARY_H */
diff --git a/meson.build b/meson.build
index 5c85a15364..c6f4b0cf5e 100644
--- a/meson.build
+++ b/meson.build
@@ -1933,7 +1933,6 @@ subdir('softmmu')
 
 common_ss.add(capstone)
 specific_ss.add(files('cpu.c', 'disas.c', 'gdbstub.c'), capstone)
-specific_ss.add(files('exec-vary.c'))
 specific_ss.add(when: 'CONFIG_TCG', if_true: files(
   'fpu/softfloat.c',
   'tcg/optimize.c',
@@ -1945,6 +1944,26 @@ specific_ss.add(when: 'CONFIG_TCG', if_true: files(
 ))
 specific_ss.add(when: 'CONFIG_TCG_INTERPRETER', if_true: files('tcg/tci.c'))
 
+# Work around a gcc bug/misfeature wherein constant propagation looks
+# through an alias:
+#   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99696
+# to guess that a const variable is always zero.  Without lto, this is
+# impossible, as the alias is restricted to page-vary-common.c.  Indeed,
+# without lto, not even the alias is required -- we simply use different
+# declarations in different compilation units.
+pagevary = files('page-vary-common.c')
+if get_option('b_lto')
+  pagevary_flags = ['-fno-lto']
+  if get_option('cfi')
+    pagevary_flags += '-fno-sanitize=cfi-icall'
+  endif
+  pagevary = static_library('page-vary-common', sources: pagevary,
+                            c_args: pagevary_flags)
+  pagevary = declare_dependency(link_with: pagevary)
+endif
+common_ss.add(pagevary)
+specific_ss.add(files('page-vary.c'))
+
 subdir('backends')
 subdir('disas')
 subdir('migration')
diff --git a/page-vary-common.c b/page-vary-common.c
new file mode 100644
index 0000000000..9175556498
--- /dev/null
+++ b/page-vary-common.c
@@ -0,0 +1,54 @@
+/*
+ * Variable page size handling -- target independent 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/>.
+ */
+
+#define IN_PAGE_VARY 1
+
+#include "qemu/osdep.h"
+#include "qemu-common.h"
+#include "exec/page-vary.h"
+
+/* WARNING: This file must *not* be complied with -flto. */
+
+TargetPageBits target_page;
+
+bool set_preferred_target_page_bits_common(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.
+     */
+    if (target_page.bits == 0 || target_page.bits > bits) {
+        if (target_page.decided) {
+            return false;
+        }
+        target_page.bits = bits;
+    }
+    return true;
+}
+
+void finalize_target_page_bits_common(int min)
+{
+    if (target_page.bits == 0) {
+        target_page.bits = min;
+    }
+    target_page.mask = -1ull << target_page.bits;
+    target_page.decided = true;
+}
diff --git a/page-vary.c b/page-vary.c
new file mode 100644
index 0000000000..057c7f1815
--- /dev/null
+++ b/page-vary.c
@@ -0,0 +1,41 @@
+/*
+ * 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/>.
+ */
+
+#define IN_PAGE_VARY 1
+
+#include "qemu/osdep.h"
+#include "qemu-common.h"
+#include "exec/exec-all.h"
+
+bool set_preferred_target_page_bits(int bits)
+{
+#ifdef TARGET_PAGE_BITS_VARY
+    assert(bits >= TARGET_PAGE_BITS_MIN);
+    return set_preferred_target_page_bits_common(bits);
+#else
+    return true;
+#endif
+}
+
+void finalize_target_page_bits(void)
+{
+#ifdef TARGET_PAGE_BITS_VARY
+    finalize_target_page_bits_common(TARGET_PAGE_BITS_MIN);
+#endif
+}
diff --git a/tcg/tcg.c b/tcg/tcg.c
index de91bb6e9e..1fbe0b686d 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -828,7 +828,6 @@ void tcg_region_init(void)
     size_t region_size;
     size_t n_regions;
     size_t i;
-    uintptr_t splitwx_diff;
 
     n_regions = tcg_n_regions();
 
@@ -858,19 +857,22 @@ void tcg_region_init(void)
     /* account for that last guard page */
     region.end -= page_size;
 
-    /* set guard pages */
-    splitwx_diff = tcg_splitwx_diff;
+    /*
+     * Set guard pages in the rw buffer, as that's the one into which
+     * buffer overruns could occur.  Do not set guard pages in the rx
+     * buffer -- let that one use hugepages throughout.
+     */
     for (i = 0; i < region.n; i++) {
         void *start, *end;
-        int rc;
 
         tcg_region_bounds(i, &start, &end);
-        rc = qemu_mprotect_none(end, page_size);
-        g_assert(!rc);
-        if (splitwx_diff) {
-            rc = qemu_mprotect_none(end + splitwx_diff, page_size);
-            g_assert(!rc);
-        }
+
+        /*
+         * macOS 11.2 has a bug (Apple Feedback FB8994773) in which mprotect
+         * rejects a permission change from RWX -> NONE.  Guard pages are
+         * nice for bug detection but are not essential; ignore any failure.
+         */
+        (void)qemu_mprotect_none(end, page_size);
     }
 
     tcg_region_trees_init();