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/*
* 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 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.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
}
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