7893e42d5d
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org> Message-ID: <20231004090629.37473-6-philmd@linaro.org> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
65 lines
2.2 KiB
C++
65 lines
2.2 KiB
C++
/*
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* Simple interface for 128-bit atomic operations.
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*
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* Copyright (C) 2018 Linaro, Ltd.
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*
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* This work is licensed under the terms of the GNU GPL, version 2 or later.
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* See the COPYING file in the top-level directory.
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*
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* See docs/devel/atomics.rst for discussion about the guarantees each
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* atomic primitive is meant to provide.
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*/
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#ifndef QEMU_ATOMIC128_H
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#define QEMU_ATOMIC128_H
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#include "qemu/int128.h"
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/*
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* If __alignof(unsigned __int128) < 16, GCC may refuse to inline atomics
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* that are supported by the host, e.g. s390x. We can force the pointer to
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* have our known alignment with __builtin_assume_aligned, however prior to
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* GCC 13 that was only reliable with optimization enabled. See
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* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107389
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*/
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#if defined(CONFIG_ATOMIC128_OPT)
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# if !defined(__OPTIMIZE__)
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# define ATTRIBUTE_ATOMIC128_OPT __attribute__((optimize("O1")))
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# endif
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# define CONFIG_ATOMIC128
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#endif
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#ifndef ATTRIBUTE_ATOMIC128_OPT
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# define ATTRIBUTE_ATOMIC128_OPT
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#endif
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/*
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* GCC is a house divided about supporting large atomic operations.
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*
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* For hosts that only have large compare-and-swap, a legalistic reading
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* of the C++ standard means that one cannot implement __atomic_read on
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* read-only memory, and thus all atomic operations must synchronize
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* through libatomic.
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*
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* See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80878
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*
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* This interpretation is not especially helpful for QEMU.
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* For system-mode, all RAM is always read/write from the hypervisor.
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* For user-mode, if the guest doesn't implement such an __atomic_read
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* then the host need not worry about it either.
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*
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* Moreover, using libatomic is not an option, because its interface is
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* built for std::atomic<T>, and requires that *all* accesses to such an
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* object go through the library. In our case we do not have an object
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* in the C/C++ sense, but a view of memory as seen by the guest.
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* The guest may issue a large atomic operation and then access those
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* pieces using word-sized accesses. From the hypervisor, we have no
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* way to connect those two actions.
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*
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* Therefore, special case each platform.
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*/
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#include "host/atomic128-cas.h"
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#include "host/atomic128-ldst.h"
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#endif /* QEMU_ATOMIC128_H */
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