[RFC PATCH 1/4] target/ppc: Fix eieio memory ordering semantics

[Nicholas Piggin <npiggin@gmail.com>]

The generated eieio memory ordering semantics do not match the
instruction definition in the architecture. Add a big comment to
explain this strange instruction and correct the memory ordering
behaviour.

Signed-off: Nicholas Piggin <npiggin@gmail.com>
---
 target/ppc/translate.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/target/ppc/translate.c b/target/ppc/translate.c
index fa34f81c30..abb8807180 100644
--- a/target/ppc/translate.c
+++ b/target/ppc/translate.c
@@ -3513,7 +3513,31 @@ static void gen_stswx(DisasContext *ctx)
 /* eieio */
 static void gen_eieio(DisasContext *ctx)
 {
-    TCGBar bar = TCG_MO_LD_ST;
+    TCGBar bar = TCG_MO_ALL;
+
+    /*
+     * eieio has complex semanitcs. It provides memory ordering between
+     * operations in the set:
+     * - loads from CI memory.
+     * - stores to CI memory.
+     * - stores to WT memory.
+     *
+     * It separately also orders memory for operations in the set:
+     * - stores to cacheble memory.
+     *
+     * It also serializes instructions:
+     * - dcbt and dcbst.
+     *
+     * It separately serializes:
+     * - tlbie and tlbsync.
+     *
+     * And separately serializes:
+     * - slbieg, slbiag, and slbsync.
+     *
+     * The end result is that CI memory ordering requires TCG_MO_ALL
+     * and it is not possible to special-case more relaxed ordering for
+     * cacheable accesses. TCG_BAR_SC is required to provide the serialization.
+     */
 
     /*
      * POWER9 has a eieio instruction variant using bit 6 as a hint to
-- 
2.35.1