LXR qemu/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 */
  44static 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
  57extern 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 */
  68TargetPageBits target_page;
  69#  define init_target_page target_page
  70# endif
  71#endif
  72
  73bool 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
  93void 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}
 109