tcg: Do not elide memory barriers for !CF_PARALLEL in system mode

The virtio devices require proper memory ordering between the vcpus and the iothreads. Reviewed-by: Philippe Mathieu-Daudé <[email protected]> Signed-off-by: Richard Henderson <[email protected]> (cherry picked from commit c914d46d0a645e7c633292146f4e38c945d4f847)
CTSRD-CHERI · Aug 17, 2023 · 4fb6c37 · 4fb6c37
1 parent 475b990
commit 4fb6c37
Showing 1 changed file with 13 additions and 1 deletion.
diff --git a/tcg/tcg-op.c b/tcg/tcg-op.c
@@ -101,7 +101,19 @@ void tcg_gen_op6(TCGOpcode opc, TCGArg a1, TCGArg a2, TCGArg a3,
 
 void tcg_gen_mb(TCGBar mb_type)
 {
-    if (tcg_ctx->tb_cflags & CF_PARALLEL) {
+#ifdef CONFIG_USER_ONLY
+    bool parallel = tcg_ctx->tb_cflags & CF_PARALLEL;
+#else
+    /*
+     * It is tempting to elide the barrier in a uniprocessor context.
+     * However, even with a single cpu we have i/o threads running in
+     * parallel, and lack of memory order can result in e.g. virtio
+     * queue entries being read incorrectly.
+     */
+    bool parallel = true;
+#endif
+
+    if (parallel) {
         tcg_gen_op1(INDEX_op_mb, mb_type);
     }
 }