s390/ipl: Fix waiting for virtio processing

The guest side must not manipulate the index for the used buffers. Instead,
remember the state of the used buffer locally and wait until it has moved.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

pc-bios/s390-ccw/virtio.c

@@ -123,6 +123,7 @@ static void vring_init(struct vring *vr, unsigned int num, void *p,
     /* We're running with interrupts off anyways, so don't bother */
     vr->used->flags = VRING_USED_F_NO_NOTIFY;
     vr->used->idx = 0;
+    vr->used_idx = 0;
 
     debug_print_addr("init vr", vr);
 }
@@ -150,8 +151,6 @@ static void vring_send_buf(struct vring *vr, void *p, int len, int flags)
     if (!(flags & VRING_DESC_F_NEXT)) {
         vr->avail->idx++;
     }
-
-    vr->used->idx = vr->next_idx;
 }
 
 static u64 get_clock(void)
@@ -180,7 +179,8 @@ static int vring_wait_reply(struct vring *vr, int timeout)
     struct subchannel_id schid = vr->schid;
     int r = 0;
 
-    while (vr->used->idx == vr->next_idx) {
+    /* Wait until the used index has moved. */
+    while (vr->used->idx == vr->used_idx) {
         vring_notify(schid);
         if (timeout && (get_second() >= target_second)) {
             r = 1;
@@ -189,6 +189,7 @@ static int vring_wait_reply(struct vring *vr, int timeout)
         yield();
     }
 
+    vr->used_idx = vr->used->idx;
     vr->next_idx = 0;
     vr->desc[0].len = 0;
     vr->desc[0].flags = 0;

pc-bios/s390-ccw/virtio.h

@@ -115,6 +115,7 @@ struct vring_used {
 struct vring {
     unsigned int num;
     int next_idx;
+    int used_idx;
     struct vring_desc *desc;
     struct vring_avail *avail;
     struct vring_used *used;