net/xen-netfront: Make it running on 64KB page granularity

net/xen-netfront: Make it running on 64KB page granularity

The PV network protocol is using 4KB page granularity. The goal of this
patch is to allow a Linux using 64KB page granularity using network
device on a non-modified Xen.

It's only necessary to adapt the ring size and break skb data in small
chunk of 4KB. The rest of the code is relying on the grant table code.

Note that we allocate a Linux page for each rx skb but only the first
4KB is used. We may improve the memory usage by extending the size of
the rx skb.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
---
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: netdev@vger.kernel.org
Improvement such as support of 64KB grant is not taken into
consideration in this patch because we have the requirement to run a Linux
using 64KB pages on a non-modified Xen.

Tested with workload such as ping, ssh, wget, git... I would happy if
someone give details how to test all the path.

    Changes in v4:
        - s/gnttab_one_grant/gnttab_for_one_grant/ based on the new naming
        - Add David's reviewed-by

    Changes in v3:
        - Fix errors reported by checkpatch.pl
        - s/mfn/gfn/ base on the new naming
        - xennet_tx_setup_grant was calling itself resulting an
        guest stall when using iperf.
        - The grant callback doesn't allow anymore to change the len
        (wasn't used here)
        - gnttab_foreach_grant has been renamed to gnttab_foreach_grant_in_range
        - gnttab_page_grant_foreign_ref has been renamed to
        gnttab_foreach_grant_foreign_ref_one

    Changes in v2:
        - Use gnttab_foreach_grant to split a Linux page in grant
        - Fix count slots