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/*
* Simple interface for 128-bit atomic operations.
*
* Copyright (C) 2018 Linaro, Ltd.
*
* This work is licensed under the terms of the GNU GPL, version 2 or later.
* See the COPYING file in the top-level directory.
*
* See docs/devel/atomics.rst for discussion about the guarantees each
* atomic primitive is meant to provide.
*/
#ifndef QEMU_ATOMIC128_H
#define QEMU_ATOMIC128_H
#include "qemu/int128.h"
/*
* If __alignof(unsigned __int128) < 16, GCC may refuse to inline atomics
* that are supported by the host, e.g. s390x. We can force the pointer to
* have our known alignment with __builtin_assume_aligned, however prior to
* GCC 13 that was only reliable with optimization enabled. See
* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107389
*/
#if defined(CONFIG_ATOMIC128_OPT)
# if !defined(__OPTIMIZE__)
# define ATTRIBUTE_ATOMIC128_OPT __attribute__((optimize("O1")))
# endif
# define CONFIG_ATOMIC128
#endif
#ifndef ATTRIBUTE_ATOMIC128_OPT
# define ATTRIBUTE_ATOMIC128_OPT
#endif
/*
* GCC is a house divided about supporting large atomic operations.
*
* For hosts that only have large compare-and-swap, a legalistic reading
* of the C++ standard means that one cannot implement __atomic_read on
* read-only memory, and thus all atomic operations must synchronize
* through libatomic.
*
* See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80878
*
* This interpretation is not especially helpful for QEMU.
* For softmmu, all RAM is always read/write from the hypervisor.
* For user-only, if the guest doesn't implement such an __atomic_read
* then the host need not worry about it either.
*
* Moreover, using libatomic is not an option, because its interface is
* built for std::atomic<T>, and requires that *all* accesses to such an
* object go through the library. In our case we do not have an object
* in the C/C++ sense, but a view of memory as seen by the guest.
* The guest may issue a large atomic operation and then access those
* pieces using word-sized accesses. From the hypervisor, we have no
* way to connect those two actions.
*
* Therefore, special case each platform.
*/
#include "host/atomic128-cas.h"
#include "host/atomic128-ldst.h"
#endif /* QEMU_ATOMIC128_H */
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