include/qemu/atomic128.h

   1 /*
   2  * Simple interface for 128-bit atomic operations.
   3  *
   4  * Copyright (C) 2018 Linaro, Ltd.
   5  *
   6  * This work is licensed under the terms of the GNU GPL, version 2 or later.
   7  * See the COPYING file in the top-level directory.
   8  *
   9  * See docs/devel/atomics.rst for discussion about the guarantees each
  10  * atomic primitive is meant to provide.
  11  */
  12
  13 #ifndef QEMU_ATOMIC128_H
  14 #define QEMU_ATOMIC128_H
  15
  16 #include "qemu/int128.h"
  17
  18 /*
  19  * If __alignof(unsigned __int128) < 16, GCC may refuse to inline atomics
  20  * that are supported by the host, e.g. s390x.  We can force the pointer to
  21  * have our known alignment with __builtin_assume_aligned, however prior to
  22  * GCC 13 that was only reliable with optimization enabled.  See
  23  *   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107389
  24  */
  25 #if defined(CONFIG_ATOMIC128_OPT)
  26 # if !defined(__OPTIMIZE__)
  27 #  define ATTRIBUTE_ATOMIC128_OPT  __attribute__((optimize("O1")))
  28 # endif
  29 # define CONFIG_ATOMIC128
  30 #endif
  31 #ifndef ATTRIBUTE_ATOMIC128_OPT
  32 # define ATTRIBUTE_ATOMIC128_OPT
  33 #endif
  34
  35 /*
  36  * GCC is a house divided about supporting large atomic operations.
  37  *
  38  * For hosts that only have large compare-and-swap, a legalistic reading
  39  * of the C++ standard means that one cannot implement __atomic_read on
  40  * read-only memory, and thus all atomic operations must synchronize
  41  * through libatomic.
  42  *
  43  * See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80878
  44  *
  45  * This interpretation is not especially helpful for QEMU.
  46  * For system-mode, all RAM is always read/write from the hypervisor.
  47  * For user-mode, if the guest doesn't implement such an __atomic_read
  48  * then the host need not worry about it either.
  49  *
  50  * Moreover, using libatomic is not an option, because its interface is
  51  * built for std::atomic<T>, and requires that *all* accesses to such an
  52  * object go through the library.  In our case we do not have an object
  53  * in the C/C++ sense, but a view of memory as seen by the guest.
  54  * The guest may issue a large atomic operation and then access those
  55  * pieces using word-sized accesses.  From the hypervisor, we have no
  56  * way to connect those two actions.
  57  *
  58  * Therefore, special case each platform.
  59  */
  60
  61 #include "host/atomic128-cas.h"
  62 #include "host/atomic128-ldst.h"
  63
  64 #endif /* QEMU_ATOMIC128_H */