tcg: Split CONFIG_ATOMIC128

GCC7+ will no longer advertise support for 16-byte __atomic operations
if only cmpxchg is supported, as for x86_64.  Fortunately, x86_64 still
has support for __sync_compare_and_swap_16 and we can make use of that.
AArch64 does not have, nor ever has had such support, so open-code it.

Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

2 files changed, 164 insertions, 0 deletions

diff --git a/include/qemu/atomic128.h b/include/qemu/atomic128.h
new file mode 100644
index 0000000..a6af22f
--- /dev/null
+++ b/include/qemu/atomic128.h
@@ -0,0 +1,153 @@
+/*
+ * 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.txt for discussion about the guarantees each
+ * atomic primitive is meant to provide.
+ */
+
+#ifndef QEMU_ATOMIC128_H
+#define QEMU_ATOMIC128_H
+
+/*
+ * 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.
+ */
+
+#if defined(CONFIG_ATOMIC128)
+static inline Int128 atomic16_cmpxchg(Int128 *ptr, Int128 cmp, Int128 new)
+{
+    return atomic_cmpxchg__nocheck(ptr, cmp, new);
+}
+# define HAVE_CMPXCHG128 1
+#elif defined(CONFIG_CMPXCHG128)
+static inline Int128 atomic16_cmpxchg(Int128 *ptr, Int128 cmp, Int128 new)
+{
+    return __sync_val_compare_and_swap_16(ptr, cmp, new);
+}
+# define HAVE_CMPXCHG128 1
+#elif defined(__aarch64__)
+/* Through gcc 8, aarch64 has no support for 128-bit at all.  */
+static inline Int128 atomic16_cmpxchg(Int128 *ptr, Int128 cmp, Int128 new)
+{
+    uint64_t cmpl = int128_getlo(cmp), cmph = int128_gethi(cmp);
+    uint64_t newl = int128_getlo(new), newh = int128_gethi(new);
+    uint64_t oldl, oldh;
+    uint32_t tmp;
+
+    asm("0: ldaxp %[oldl], %[oldh], %[mem]\n\t"
+        "cmp %[oldl], %[cmpl]\n\t"
+        "ccmp %[oldh], %[cmph], #0, eq\n\t"
+        "b.ne 1f\n\t"
+        "stlxp %w[tmp], %[newl], %[newh], %[mem]\n\t"
+        "cbnz %w[tmp], 0b\n"
+        "1:"
+        : [mem] "+m"(*ptr), [tmp] "=&r"(tmp),
+          [oldl] "=&r"(oldl), [oldh] "=r"(oldh)
+        : [cmpl] "r"(cmpl), [cmph] "r"(cmph),
+          [newl] "r"(newl), [newh] "r"(newh)
+        : "memory", "cc");
+
+    return int128_make128(oldl, oldh);
+}
+# define HAVE_CMPXCHG128 1
+#else
+/* Fallback definition that must be optimized away, or error.  */
+Int128 QEMU_ERROR("unsupported atomic")
+    atomic16_cmpxchg(Int128 *ptr, Int128 cmp, Int128 new);
+# define HAVE_CMPXCHG128 0
+#endif /* Some definition for HAVE_CMPXCHG128 */
+
+
+#if defined(CONFIG_ATOMIC128)
+static inline Int128 atomic16_read(Int128 *ptr)
+{
+    return atomic_read__nocheck(ptr);
+}
+
+static inline void atomic16_set(Int128 *ptr, Int128 val)
+{
+    atomic_set__nocheck(ptr, val);
+}
+
+# define HAVE_ATOMIC128 1
+#elif !defined(CONFIG_USER_ONLY) && defined(__aarch64__)
+/* We can do better than cmpxchg for AArch64.  */
+static inline Int128 atomic16_read(Int128 *ptr)
+{
+    uint64_t l, h;
+    uint32_t tmp;
+
+    /* The load must be paired with the store to guarantee not tearing.  */
+    asm("0: ldxp %[l], %[h], %[mem]\n\t"
+        "stxp %w[tmp], %[l], %[h], %[mem]\n\t"
+        "cbnz %w[tmp], 0b"
+        : [mem] "+m"(*ptr), [tmp] "=r"(tmp), [l] "=r"(l), [h] "=r"(h));
+
+    return int128_make128(l, h);
+}
+
+static inline void atomic16_set(Int128 *ptr, Int128 val)
+{
+    uint64_t l = int128_getlo(val), h = int128_gethi(val);
+    uint64_t t1, t2;
+
+    /* Load into temporaries to acquire the exclusive access lock.  */
+    asm("0: ldxp %[t1], %[t2], %[mem]\n\t"
+        "stxp %w[t1], %[l], %[h], %[mem]\n\t"
+        "cbnz %w[t1], 0b"
+        : [mem] "+m"(*ptr), [t1] "=&r"(t1), [t2] "=&r"(t2)
+        : [l] "r"(l), [h] "r"(h));
+}
+
+# define HAVE_ATOMIC128 1
+#elif !defined(CONFIG_USER_ONLY) && HAVE_CMPXCHG128
+static inline Int128 atomic16_read(Int128 *ptr)
+{
+    /* Maybe replace 0 with 0, returning the old value.  */
+    return atomic16_cmpxchg(ptr, 0, 0);
+}
+
+static inline void atomic16_set(Int128 *ptr, Int128 val)
+{
+    Int128 old = *ptr, cmp;
+    do {
+        cmp = old;
+        old = atomic16_cmpxchg(ptr, cmp, val);
+    } while (old != cmp);
+}
+
+# define HAVE_ATOMIC128 1
+#else
+/* Fallback definitions that must be optimized away, or error.  */
+Int128 QEMU_ERROR("unsupported atomic") atomic16_read(Int128 *ptr);
+void QEMU_ERROR("unsupported atomic") atomic16_set(Int128 *ptr, Int128 val);
+# define HAVE_ATOMIC128 0
+#endif /* Some definition for HAVE_ATOMIC128 */
+
+#endif /* QEMU_ATOMIC128_H */
diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
index 66f8c24..6b92710 100644
--- a/include/qemu/compiler.h
+++ b/include/qemu/compiler.h
@@ -146,6 +146,17 @@
 # define QEMU_FLATTEN
 #endif
 
+/*
+ * If __attribute__((error)) is present, use it to produce an error at
+ * compile time.  Otherwise, one must wait for the linker to diagnose
+ * the missing symbol.
+ */
+#if __has_attribute(error)
+# define QEMU_ERROR(X) __attribute__((error(X)))
+#else
+# define QEMU_ERROR(X)
+#endif
+
 /* Implement C11 _Generic via GCC builtins.  Example:
  *
  *    QEMU_GENERIC(x, (float, sinf), (long double, sinl), sin) (x)