s390/ipl: Fix spurious errors in virtio

With the ccw ipl code sometimes an error message like
"virtio: trying to map MMIO memory" or
"Guest moved used index from %u to %u" appeared. Turns out
that the ccw bios did not zero out the vring, which might
cause stale values in avail->idx and friends, especially
on reboot.

Lets zero out the relevant fields. To activate the patch we
need to rebuild s390-ccw.img as well.
Signed-off-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Message-id: 1369309901-418-1-git-send-email-borntraeger@de.ibm.com
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

pc-bios/s390-ccw/virtio.c

@@ -114,8 +114,13 @@ static void vring_init(struct vring *vr, unsigned int num, void *p,
     vr->used = (void *)(((unsigned long)&vr->avail->ring[num] + align-1)
                 & ~(align - 1));
 
+    /* Zero out all relevant field */
+    vr->avail->flags = 0;
+    vr->avail->idx = 0;
+
     /* We're running with interrupts off anyways, so don't bother */
     vr->used->flags = VRING_USED_F_NO_NOTIFY;
+    vr->used->idx = 0;
 
     debug_print_addr("init vr", vr);
 }