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Re: [Xen-devel] [PATCH net-next v2 2/2] xen-netback: Grant copy the header instead of map and memcpy



On 01/04/14 12:40, Ian Campbell wrote:
On Mon, 2014-03-31 at 16:08 +0100, Zoltan Kiss wrote:

  check_frags:
-       for (i = start; i < nr_frags; i++) {
+       for (i = 0; i < nr_frags; i++, gop_map++) {
                int j, newerr;

                pending_idx = frag_get_pending_idx(&shinfo->frags[i]);
-               tx_info = &vif->pending_tx_info[pending_idx];

                /* Check error status: if okay then remember grant handle. */
-               newerr = (++gop_map)->status;
+               newerr = (gop_map)->status;

You've reworked the handling of gop_map and when and where it is
incremented, which might be a legit cleanup but does it relate to the
bulk of this change somehow that I'm missing?
That original "++gop_map" assumed the header was also grant mapped, and incremented the pointer first here, which is wrong now.


  [...]
                __skb_put(skb, data_len);
+               vif->tx_copy_ops[*copy_ops].source.u.ref = txreq.gref;
+               vif->tx_copy_ops[*copy_ops].source.domid = vif->domid;
+               vif->tx_copy_ops[*copy_ops].source.offset = txreq.offset;
+
+               vif->tx_copy_ops[*copy_ops].dest.u.gmfn =
+                       virt_to_mfn(skb->data);
+               vif->tx_copy_ops[*copy_ops].dest.domid = DOMID_SELF;
+               vif->tx_copy_ops[*copy_ops].dest.offset =
+                       offset_in_page(skb->data);
+
+               vif->tx_copy_ops[*copy_ops].len = data_len;
+               vif->tx_copy_ops[*copy_ops].flags = GNTCOPY_source_gref;

We have gnttab_set_map_op. Should we have gnttap_set_copy_op too?
This is the only place at the moment when we do this, so I wouldn't bother to do it.


-       BUG_ON(ret);
+       else {
+               gnttab_batch_copy(vif->tx_copy_ops, nr_cops);
+               if (nr_mops != 0) {


if (nr_mops) would do.

+                       ret = gnttab_map_refs(vif->tx_map_ops,

So we use gnttab_batch_copy and gnttab_map_refs.

Shouldn't we either use gnttab_batch_copy and gnttab_batch_map or
gnttab_copy gnttab_map_refs. (where gnttab_copy might be a bare
GNTTABOP_copy or might be a helper wrapper).

The point of the batch interface is to handle page unsharing etc, but
doing it only for copies seems like a waste one way or another.
The difference between gnttab_batch_map and gnttab_map_refs is that the latter calls set_foreign_p2m_mapping, which we need for sure. gnttab_batch_copy calls the hypercall and tries again if op->status == GNTST_eagain. I think that's exactly what we need here as well. The naming might be confusing indeed, but that should be the topic of an another patch.

Zoli

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