[PATCH 2/5] tcg: Do not elide memory barriers for CF_PARALLEL

[Richard Henderson]

The virtio devices require proper memory ordering between
the vcpus and the iothreads.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 tcg/tcg-op.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/tcg/tcg-op.c b/tcg/tcg-op.c
index 70475773f4..76dc7d8dc5 100644
--- a/tcg/tcg-op.c
+++ b/tcg/tcg-op.c
@@ -97,9 +97,13 @@ 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) {
-        tcg_gen_op1(INDEX_op_mb, mb_type);
-    }
+    /*
+     * It is tempting to elide the barrier in a single-threaded context
+     * (i.e. !(tb_cflags & CF_PARALLEL)), 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.
+     */
+    tcg_gen_op1(INDEX_op_mb, mb_type);
 }
 
 /* 32 bit ops */
-- 
2.25.1