docs: XDP update requirements

Added "Header push and pop" and "RX bulking"

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

kernel/Documentation/networking/XDP/design/requirements.rst

@@ -27,21 +27,44 @@ into the stack again).
 Write access to packet-data
 ===========================
 
-Need the ability to modify packet-data.  This is unfortunately
+Need the ability to modify packet-data.  This is unfortunately often
 difficult to obtain, as it requires fundamental changes to the drivers
 memory model.
 
 Unfortunately most driver don't have "writable" packet-data as
-default.  The packet-data is not writable, because choose to
-work-arounds performance bottlenecks in both the page-allocator and
-DMA APIs, which side-effect is read-only packet-pages.
+default.  The packet-data in drivers is often not writable, because
+the drivers likely have choosen to work-arounds performance
+bottlenecks in both the page-allocator and DMA APIs, which side-effect
+is read-only packet-pages.
 
 Instead most drivers, allocate both a SKB and a writable memory
 buffer, which the packet headers are copied into (and for placing
-``skb_shared_info``). Then, the SKB (and ``skb_shared_info``) is
+``skb_shared_info``). Afterwards the SKB (and ``skb_shared_info``) is
 adjusted to point into the remaining payload (pointing past the
 headers just copied).
 
+
+Header push and pop
+===================
+
+The ability to push (add) or pop (remove) packet headers indirectly
+depend on write acces to packet-data. (One could argue that a pure
+pop, could be implemented by only adjusting the payload offset, thus
+no write-access).
+
+This requirement goes hand-in-hand with tunnel encapsulation or
+decapsulation.  It is also relevant for e.g adding a VLAN head as
+needed by the :doc:`../use-cases/xdp_use_case_ddos_scrubber` in-order
+to workaround the :ref:`XDP_TX` single NIC limitation.
+
+This requirement implies the ability to adjust the packet-data start
+offset/pointer and packet length.  This requires additional data to be
+returned
+
+This also have implication for how much headroom drivers should
+reserve.
+
+
 Page per packet
 ===============
 
@@ -54,3 +77,6 @@ demonstrating superior performance.  For the shear ability to compare
 against DPDK, XDP also need a forwarding capability.
 
 
+RX bulking
+==========
+