qemu/atomic128: Split atomic16_read

Create both atomic16_read_ro and atomic16_read_rw.
Previously we pretended that we had atomic16_read in system mode,
because we "know" that all ram is always writable to the host.
Now, expose read-only and read-write versions all of the time.

For aarch64, do not fall back to __atomic_read_16 even if
supported by the compiler, to work around a clang bug.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

host/include/aarch64/host/atomic128-ldst.h

@@ -11,10 +11,18 @@
 #ifndef AARCH64_ATOMIC128_LDST_H
 #define AARCH64_ATOMIC128_LDST_H
 
-/* Through gcc 10, aarch64 has no support for 128-bit atomics.  */
-#if !defined(CONFIG_ATOMIC128) && !defined(CONFIG_USER_ONLY)
-/* We can do better than cmpxchg for AArch64.  */
-static inline Int128 atomic16_read(Int128 *ptr)
+/*
+ * Through gcc 10, aarch64 has no support for 128-bit atomics.
+ * Through clang 16, without -march=armv8.4-a, __atomic_load_16
+ * is incorrectly expanded to a read-write operation.
+ */
+
+#define HAVE_ATOMIC128_RO 0
+#define HAVE_ATOMIC128_RW 1
+
+Int128 QEMU_ERROR("unsupported atomic") atomic16_read_ro(const Int128 *ptr);
+
+static inline Int128 atomic16_read_rw(Int128 *ptr)
 {
     uint64_t l, h;
     uint32_t tmp;
@@ -41,9 +49,4 @@ static inline void atomic16_set(Int128 *ptr, Int128 val)
         : [l] "r"(l), [h] "r"(h));
 }
 
-# define HAVE_ATOMIC128 1
-#else
-#include "host/include/generic/host/atomic128-ldst.h"
-#endif
-
 #endif /* AARCH64_ATOMIC128_LDST_H */

host/include/generic/host/atomic128-ldst.h

@@ -12,16 +12,25 @@
 #define HOST_ATOMIC128_LDST_H
 
 #if defined(CONFIG_ATOMIC128)
+# define HAVE_ATOMIC128_RO 1
+# define HAVE_ATOMIC128_RW 1
+
 static inline Int128 ATTRIBUTE_ATOMIC128_OPT
-atomic16_read(Int128 *ptr)
+atomic16_read_ro(const Int128 *ptr)
 {
-    __int128_t *ptr_align = __builtin_assume_aligned(ptr, 16);
+    const __int128_t *ptr_align = __builtin_assume_aligned(ptr, 16);
     Int128Alias r;
 
     r.i = qatomic_read__nocheck(ptr_align);
     return r.s;
 }
 
+static inline Int128 ATTRIBUTE_ATOMIC128_OPT
+atomic16_read_rw(Int128 *ptr)
+{
+    return atomic16_read_ro(ptr);
+}
+
 static inline void ATTRIBUTE_ATOMIC128_OPT
 atomic16_set(Int128 *ptr, Int128 val)
 {
@@ -32,10 +41,14 @@ atomic16_set(Int128 *ptr, Int128 val)
     qatomic_set__nocheck(ptr_align, v.i);
 }
 
-# define HAVE_ATOMIC128 1
-#elif defined(CONFIG_CMPXCHG128) && !defined(CONFIG_USER_ONLY)
+#elif defined(CONFIG_CMPXCHG128)
+# define HAVE_ATOMIC128_RO 0
+# define HAVE_ATOMIC128_RW 1
+
+Int128 QEMU_ERROR("unsupported atomic") atomic16_read_ro(const Int128 *ptr);
+
 static inline Int128 ATTRIBUTE_ATOMIC128_OPT
-atomic16_read(Int128 *ptr)
+atomic16_read_rw(Int128 *ptr)
 {
     /* Maybe replace 0 with 0, returning the old value.  */
     Int128 z = int128_make64(0);
@@ -52,12 +65,14 @@ atomic16_set(Int128 *ptr, Int128 val)
     } while (int128_ne(old, cmp));
 }
 
-# define HAVE_ATOMIC128 1
 #else
+# define HAVE_ATOMIC128_RO 0
+# define HAVE_ATOMIC128_RW 0
+
 /* Fallback definitions that must be optimized away, or error.  */
-Int128 QEMU_ERROR("unsupported atomic") atomic16_read(Int128 *ptr);
+Int128 QEMU_ERROR("unsupported atomic") atomic16_read_ro(const Int128 *ptr);
+Int128 QEMU_ERROR("unsupported atomic") atomic16_read_rw(Int128 *ptr);
 void QEMU_ERROR("unsupported atomic") atomic16_set(Int128 *ptr, Int128 val);
-# define HAVE_ATOMIC128 0
 #endif
 
 #endif /* HOST_ATOMIC128_LDST_H */

target/s390x/tcg/mem_helper.c

@@ -1780,7 +1780,7 @@ static uint32_t do_csst(CPUS390XState *env, uint32_t r3, uint64_t a1,
         max = 3;
 #endif
         if ((HAVE_CMPXCHG128 ? 0 : fc + 2 > max) ||
-            (HAVE_ATOMIC128  ? 0 : sc > max)) {
+            (HAVE_ATOMIC128_RW ? 0 : sc > max)) {
             cpu_loop_exit_atomic(env_cpu(env), ra);
         }
     }