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 softmmu, all RAM is always read/write from the hypervisor. 47 * For user-only, 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 */ 65