[PATCH 5/8] xen/netfront: read response from backend only once

[Juergen Gross <jgross@xxxxxxxx>]

In order to avoid problems in case the backend is modifying a response
on the ring page while the frontend has already seen it, just read the
response into a local buffer in one go and then operate on that buffer
only.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 drivers/net/xen-netfront.c | 38 +++++++++++++++++++-------------------
 1 file changed, 19 insertions(+), 19 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 44275908d61a..f91e41ece554 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -399,13 +399,13 @@ static void xennet_tx_buf_gc(struct netfront_queue *queue)
                rmb(); /* Ensure we see responses up to 'rp'. */
 
                for (cons = queue->tx.rsp_cons; cons != prod; cons++) {
-                       struct xen_netif_tx_response *txrsp;
+                       struct xen_netif_tx_response txrsp;
 
-                       txrsp = RING_GET_RESPONSE(&queue->tx, cons);
-                       if (txrsp->status == XEN_NETIF_RSP_NULL)
+                       RING_COPY_RESPONSE(&queue->tx, cons, &txrsp);
+                       if (txrsp.status == XEN_NETIF_RSP_NULL)
                                continue;
 
-                       id  = txrsp->id;
+                       id  = txrsp.id;
                        skb = queue->tx_skbs[id].skb;
                        if (unlikely(gnttab_query_foreign_access(
                                queue->grant_tx_ref[id]) != 0)) {
@@ -814,7 +814,7 @@ static int xennet_get_extras(struct netfront_queue *queue,
                             RING_IDX rp)
 
 {
-       struct xen_netif_extra_info *extra;
+       struct xen_netif_extra_info extra;
        struct device *dev = &queue->info->netdev->dev;
        RING_IDX cons = queue->rx.rsp_cons;
        int err = 0;
@@ -830,24 +830,22 @@ static int xennet_get_extras(struct netfront_queue *queue,
                        break;
                }
 
-               extra = (struct xen_netif_extra_info *)
-                       RING_GET_RESPONSE(&queue->rx, ++cons);
+               RING_COPY_RESPONSE(&queue->rx, ++cons, &extra);
 
-               if (unlikely(!extra->type ||
-                            extra->type >= XEN_NETIF_EXTRA_TYPE_MAX)) {
+               if (unlikely(!extra.type ||
+                            extra.type >= XEN_NETIF_EXTRA_TYPE_MAX)) {
                        if (net_ratelimit())
                                dev_warn(dev, "Invalid extra type: %d\n",
-                                       extra->type);
+                                       extra.type);
                        err = -EINVAL;
                } else {
-                       memcpy(&extras[extra->type - 1], extra,
-                              sizeof(*extra));
+                       memcpy(&extras[extra.type - 1], &extra, sizeof(extra));
                }
 
                skb = xennet_get_rx_skb(queue, cons);
                ref = xennet_get_rx_ref(queue, cons);
                xennet_move_rx_slot(queue, skb, ref);
-       } while (extra->flags & XEN_NETIF_EXTRA_FLAG_MORE);
+       } while (extra.flags & XEN_NETIF_EXTRA_FLAG_MORE);
 
        queue->rx.rsp_cons = cons;
        return err;
@@ -905,7 +903,7 @@ static int xennet_get_responses(struct netfront_queue 
*queue,
                                struct sk_buff_head *list,
                                bool *need_xdp_flush)
 {
-       struct xen_netif_rx_response *rx = &rinfo->rx;
+       struct xen_netif_rx_response *rx = &rinfo->rx, rx_local;
        int max = XEN_NETIF_NR_SLOTS_MIN + (rx->status <= RX_COPY_THRESHOLD);
        RING_IDX cons = queue->rx.rsp_cons;
        struct sk_buff *skb = xennet_get_rx_skb(queue, cons);
@@ -989,7 +987,8 @@ static int xennet_get_responses(struct netfront_queue 
*queue,
                        break;
                }
 
-               rx = RING_GET_RESPONSE(&queue->rx, cons + slots);
+               RING_COPY_RESPONSE(&queue->rx, cons + slots, &rx_local);
+               rx = &rx_local;
                skb = xennet_get_rx_skb(queue, cons + slots);
                ref = xennet_get_rx_ref(queue, cons + slots);
                slots++;
@@ -1044,10 +1043,11 @@ static int xennet_fill_frags(struct netfront_queue 
*queue,
        struct sk_buff *nskb;
 
        while ((nskb = __skb_dequeue(list))) {
-               struct xen_netif_rx_response *rx =
-                       RING_GET_RESPONSE(&queue->rx, ++cons);
+               struct xen_netif_rx_response rx;
                skb_frag_t *nfrag = &skb_shinfo(nskb)->frags[0];
 
+               RING_COPY_RESPONSE(&queue->rx, ++cons, &rx);
+
                if (skb_shinfo(skb)->nr_frags == MAX_SKB_FRAGS) {
                        unsigned int pull_to = NETFRONT_SKB_CB(skb)->pull_to;
 
@@ -1062,7 +1062,7 @@ static int xennet_fill_frags(struct netfront_queue *queue,
 
                skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags,
                                skb_frag_page(nfrag),
-                               rx->offset, rx->status, PAGE_SIZE);
+                               rx.offset, rx.status, PAGE_SIZE);
 
                skb_shinfo(nskb)->nr_frags = 0;
                kfree_skb(nskb);
@@ -1161,7 +1161,7 @@ static int xennet_poll(struct napi_struct *napi, int 
budget)
        i = queue->rx.rsp_cons;
        work_done = 0;
        while ((i != rp) && (work_done < budget)) {
-               memcpy(rx, RING_GET_RESPONSE(&queue->rx, i), sizeof(*rx));
+               RING_COPY_RESPONSE(&queue->rx, i, rx);
                memset(extras, 0, sizeof(rinfo.extras));
 
                err = xennet_get_responses(queue, &rinfo, rp, &tmpq,
-- 
2.26.2