[Qemu-devel] [PATCH v2] target/ppc: extend eieio for POWER9

["Cédric Le Goater" <clg@kaod.org>]

POWER9 introduced a new variant of the eieio instruction using bit 6
as a hint to tell the CPU it is a store-forwarding barrier.

The usage of this eieio extension was recently added in Linux 4.17
which activated the "support for a store forwarding barrier at kernel
entry/exit".

Unfortunately, it is not possible to insert this new eieio instruction
without considerable change in ppc_tr_translate_insn(). So instead we
loosen the QEMU eieio instruction mask and modify the gen_eieio()
helper to test for bit6. On non-POWER9 CPUs, the bit6 is just ignored
but a warning is emitted as this is not an instruction software should
be using.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
---

 Changes since v1:

 - removed specific PPC2_MEM_EIEIO2 flag
 - ignore bit6 on non-POWER9 CPU

 target/ppc/translate.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/target/ppc/translate.c b/target/ppc/translate.c
index 8ba8f67dc513..5fe1ba655599 100644
--- a/target/ppc/translate.c
+++ b/target/ppc/translate.c
@@ -2967,7 +2967,28 @@ static void gen_stswx(DisasContext *ctx)
 /* eieio */
 static void gen_eieio(DisasContext *ctx)
 {
-    tcg_gen_mb(TCG_MO_LD_ST | TCG_BAR_SC);
+    TCGBar bar = TCG_MO_LD_ST;
+
+    /*
+     * POWER9 has a eieio instruction variant using bit 6 as a hint to
+     * tell the CPU it is a store-forwarding barrier.
+     */
+    if (ctx->opcode & 0x2000000) {
+        /*
+         * ISA says that "Reserved fields in instructions are ignored
+         * by the processor". So ignore the bit 6 on non-POWER9 CPU but
+         * as this is not an instruction software should be using,
+         * complain to the user.
+         */
+        if (!(ctx->insns_flags2 & PPC2_ISA300)) {
+            qemu_log_mask(LOG_GUEST_ERROR, "invalid eieio using bit 6 at @"
+                          TARGET_FMT_lx "\n", ctx->base.pc_next - 4);
+        } else {
+            bar = TCG_MO_ST_LD;
+        }
+    }
+
+    tcg_gen_mb(bar | TCG_BAR_SC);
 }
 
 #if !defined(CONFIG_USER_ONLY)
@@ -6483,7 +6504,7 @@ GEN_HANDLER(lswi, 0x1F, 0x15, 0x12, 0x00000001, PPC_STRING),
 GEN_HANDLER(lswx, 0x1F, 0x15, 0x10, 0x00000001, PPC_STRING),
 GEN_HANDLER(stswi, 0x1F, 0x15, 0x16, 0x00000001, PPC_STRING),
 GEN_HANDLER(stswx, 0x1F, 0x15, 0x14, 0x00000001, PPC_STRING),
-GEN_HANDLER(eieio, 0x1F, 0x16, 0x1A, 0x03FFF801, PPC_MEM_EIEIO),
+GEN_HANDLER(eieio, 0x1F, 0x16, 0x1A, 0x01FFF801, PPC_MEM_EIEIO),
 GEN_HANDLER(isync, 0x13, 0x16, 0x04, 0x03FFF801, PPC_MEM),
 GEN_HANDLER_E(lbarx, 0x1F, 0x14, 0x01, 0, PPC_NONE, PPC2_ATOMIC_ISA206),
 GEN_HANDLER_E(lharx, 0x1F, 0x14, 0x03, 0, PPC_NONE, PPC2_ATOMIC_ISA206),
-- 
2.13.6