exec-vary.c

   1 /*
   2  * Variable page size handling
   3  *
   4  *  Copyright (c) 2003 Fabrice Bellard
   5  *
   6  * This library is free software; you can redistribute it and/or
   7  * modify it under the terms of the GNU Lesser General Public
   8  * License as published by the Free Software Foundation; either
   9  * version 2 of the License, or (at your option) any later version.
  10  *
  11  * This library is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14  * Lesser General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU Lesser General Public
  17  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  18  */
  19
  20 #include "qemu/osdep.h"
  21 #include "qemu-common.h"
  22
  23 #define IN_EXEC_VARY 1
  24
  25 #include "exec/exec-all.h"
  26
  27 #ifdef TARGET_PAGE_BITS_VARY
  28 # ifdef CONFIG_ATTRIBUTE_ALIAS
  29 /*
  30  * We want to declare the "target_page" variable as const, which tells
  31  * the compiler that it can cache any value that it reads across calls.
  32  * This avoids multiple assertions and multiple reads within any one user.
  33  *
  34  * This works because we finish initializing the data before we ever read
  35  * from the "target_page" symbol.
  36  *
  37  * This also requires that we have a non-constant symbol by which we can
  38  * perform the actual initialization, and which forces the data to be
  39  * allocated within writable memory.  Thus "init_target_page", and we use
  40  * that symbol exclusively in the two functions that initialize this value.
  41  *
  42  * The "target_page" symbol is created as an alias of "init_target_page".
  43  */
  44 static TargetPageBits init_target_page;
  45
  46 /*
  47  * Note that this is *not* a redundant decl, this is the definition of
  48  * the "target_page" symbol.  The syntax for this definition requires
  49  * the use of the extern keyword.  This seems to be a GCC bug in
  50  * either the syntax for the alias attribute or in -Wredundant-decls.
  51  *
  52  * See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91765
  53  */
  54 #  pragma GCC diagnostic push
  55 #  pragma GCC diagnostic ignored "-Wredundant-decls"
  56
  57 extern const TargetPageBits target_page
  58     __attribute__((alias("init_target_page")));
  59
  60 #  pragma GCC diagnostic pop
  61 # else
  62 /*
  63  * When aliases are not supported then we force two different declarations,
  64  * by way of suppressing the header declaration with IN_EXEC_VARY.
  65  * We assume that on such an old compiler, LTO cannot be used, and so the
  66  * compiler cannot not detect the mismatched declarations, and all is well.
  67  */
  68 TargetPageBits target_page;
  69 #  define init_target_page target_page
  70 # endif
  71 #endif
  72
  73 bool set_preferred_target_page_bits(int bits)
  74 {
  75     /*
  76      * The target page size is the lowest common denominator for all
  77      * the CPUs in the system, so we can only make it smaller, never
  78      * larger. And we can't make it smaller once we've committed to
  79      * a particular size.
  80      */
  81 #ifdef TARGET_PAGE_BITS_VARY
  82     assert(bits >= TARGET_PAGE_BITS_MIN);
  83     if (init_target_page.bits == 0 || init_target_page.bits > bits) {
  84         if (init_target_page.decided) {
  85             return false;
  86         }
  87         init_target_page.bits = bits;
  88     }
  89 #endif
  90     return true;
  91 }
  92
  93 void finalize_target_page_bits(void)
  94 {
  95 #ifdef TARGET_PAGE_BITS_VARY
  96     if (init_target_page.bits == 0) {
  97         init_target_page.bits = TARGET_PAGE_BITS_MIN;
  98     }
  99     init_target_page.mask = (target_long)-1 << init_target_page.bits;
 100     init_target_page.decided = true;
 101
 102     /*
 103      * For the benefit of an -flto build, prevent the compiler from
 104      * hoisting a read from target_page before we finish initializing.
 105      */
 106     barrier();
 107 #endif
 108 }