[PATCH] pc-bios/s390-ccw: Use memory barriers in virtio code

[Thomas Huth]

According to the virtio specification, a memory barrier should be
used before incrementing the idx field in the "available" ring.
So far, we did not do this in the s390-ccw bios yet, but recently
Peter Maydell saw problems with the s390-ccw bios when running
the qtests on an aarch64 host (the bios panic'ed with the message:
"SCSI cannot report LUNs: response VS RESP=09"), which could
maybe be related to the missing memory barriers. Thus let's add
those barriers now. Since we've only seen the problem on TCG so far,
a "bcr 14,0" should be sufficient here to trigger the tcg_gen_mb()
in the TCG translate code.

(Note: The virtio spec also talks about using a memory barrier
*after* incrementing the idx field, but if I understood correctly
this is only required when using notification suppression - which
we don't use in the s390-ccw bios here)

Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 pc-bios/s390-ccw/virtio-net.c | 1 +
 pc-bios/s390-ccw/virtio.c     | 1 +
 pc-bios/s390-ccw/virtio.h     | 2 ++
 3 files changed, 4 insertions(+)

diff --git a/pc-bios/s390-ccw/virtio-net.c b/pc-bios/s390-ccw/virtio-net.c
index 2fcb0a58c5..25598a7a97 100644
--- a/pc-bios/s390-ccw/virtio-net.c
+++ b/pc-bios/s390-ccw/virtio-net.c
@@ -127,6 +127,7 @@ int recv(int fd, void *buf, int maxlen, int flags)
 
     /* Mark buffer as available to the host again */
     rxvq->avail->ring[rxvq->avail->idx % rxvq->num] = id;
+    virtio_mb();
     rxvq->avail->idx = rxvq->avail->idx + 1;
     vring_notify(rxvq);
 
diff --git a/pc-bios/s390-ccw/virtio.c b/pc-bios/s390-ccw/virtio.c
index ab49840db8..fb9687f9b3 100644
--- a/pc-bios/s390-ccw/virtio.c
+++ b/pc-bios/s390-ccw/virtio.c
@@ -154,6 +154,7 @@ void vring_send_buf(VRing *vr, void *p, int len, int flags)
 
     /* Chains only have a single ID */
     if (!(flags & VRING_DESC_F_NEXT)) {
+        virtio_mb();
         vr->avail->idx++;
     }
 }
diff --git a/pc-bios/s390-ccw/virtio.h b/pc-bios/s390-ccw/virtio.h
index 19fceb6495..6ac65482a9 100644
--- a/pc-bios/s390-ccw/virtio.h
+++ b/pc-bios/s390-ccw/virtio.h
@@ -271,6 +271,8 @@ struct VirtioCmd {
 };
 typedef struct VirtioCmd VirtioCmd;
 
+#define virtio_mb()  asm volatile("bcr 14,0" : : : "memory")
+
 bool vring_notify(VRing *vr);
 int drain_irqs(SubChannelId schid);
 void vring_send_buf(VRing *vr, void *p, int len, int flags);
-- 
2.27.0