qemu/atomic128: Add runtime test for FEAT_LSE2

With FEAT_LSE2, load and store of int128 is directly supported.

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

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

@@ -11,27 +11,48 @@
 #ifndef AARCH64_ATOMIC128_LDST_H
 #define AARCH64_ATOMIC128_LDST_H
 
+#include "host/cpuinfo.h"
+#include "tcg/debug-assert.h"
+
 /*
  * 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.
+ *
+ * Anyway, this method allows runtime detection of FEAT_LSE2.
  */
 
-#define HAVE_ATOMIC128_RO 0
+#define HAVE_ATOMIC128_RO (cpuinfo & CPUINFO_LSE2)
 #define HAVE_ATOMIC128_RW 1
 
-Int128 QEMU_ERROR("unsupported atomic") atomic16_read_ro(const Int128 *ptr);
+static inline Int128 atomic16_read_ro(const Int128 *ptr)
+{
+    uint64_t l, h;
+
+    tcg_debug_assert(HAVE_ATOMIC128_RO);
+    /* With FEAT_LSE2, 16-byte aligned LDP is atomic. */
+    asm("ldp %[l], %[h], %[mem]"
+        : [l] "=r"(l), [h] "=r"(h) : [mem] "m"(*ptr));
+
+    return int128_make128(l, h);
+}
 
 static inline Int128 atomic16_read_rw(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));
+    if (cpuinfo & CPUINFO_LSE2) {
+        /* With FEAT_LSE2, 16-byte aligned LDP is atomic. */
+        asm("ldp %[l], %[h], %[mem]"
+            : [l] "=r"(l), [h] "=r"(h) : [mem] "m"(*ptr));
+    } else {
+        /* 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);
 }
@@ -41,12 +62,18 @@ 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));
+    if (cpuinfo & CPUINFO_LSE2) {
+        /* With FEAT_LSE2, 16-byte aligned STP is atomic. */
+        asm("stp %[l], %[h], %[mem]"
+            : [mem] "=m"(*ptr) : [l] "r"(l), [h] "r"(h));
+    } else {
+        /* 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));
+    }
 }
 
 #endif /* AARCH64_ATOMIC128_LDST_H */