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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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?

>  [...] 
>               __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?

> -     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.

#include <paul's-comments>

Ian.


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