Merge branch 'bpf-xsk-fix-mixed-mode'

Magnus Karlsson says:

====================
Previously, the xsk code did not record which umem was bound to a
specific queue id. This was not required if all drivers were zero-copy
enabled as this had to be recorded in the driver anyway. So if a user
tried to bind two umems to the same queue, the driver would say
no. But if copy-mode was first enabled and then zero-copy mode (or the
reverse order), we mistakenly enabled both of them on the same umem
leading to buggy behavior. The main culprit for this is that we did
not store the association of umem to queue id in the copy case and
only relied on the driver reporting this. As this relation was not
stored in the driver for copy mode (it does not rely on the AF_XDP
NDOs), this obviously could not work.

This patch fixes the problem by always recording the umem to queue id
relationship in the netdev_queue and netdev_rx_queue structs. This way
we always know what kind of umem has been bound to a queue id and can
act appropriately at bind time. To make the bind semantics consistent
with ethtool queue manipulations and to facilitate the implementation
of drivers, we also forbid decreasing the number of queues/channels
with ethtool if there is an active AF_XDP socket in the set of queues
that are disabled.

Jakub, please take a look at your patches. The last one I had to
change slightly to make it fit with the new interface
xdp_get_umem_from_qid(). An added bonus with this function is that we,
in the future, can also use it from the driver to get a umem, thus
simplifying driver implementations (and later remove the umem from the
NDO completely). Björn will mail patches, at a later point in time,
using this in the i40e and ixgbe drivers, that removes a good chunk of
code from the ZC implementations. I also made your code aware of Tx
queues. If we create a socket that only has a Tx queue, then the queue
id will refer to a Tx queue id only and could be larger than the
available amount of Rx queues. Please take a look at it.

Differences against v1:
* Included patches from Jakub that forbids decreasing the number of active
  queues if a queue to be deactivated has an AF_XDP socket. These have
  been adapted somewhat to the new interfaces in patch 2.
* Removed redundant check against real_num_[rt]x_queue in xsk_bind
* Only need to test against real_num_[rt]x_queues in
  xdp_clear_umem_at_qid.

Patch 1: Introduces a umem reference in the netdev_rx_queue and
         netdev_queue structs.
Patch 2: Records which queue_id is bound to which umem and make sure
         that you cannot bind two different umems to the same queue_id.
Patch 3: Pre patch to ethtool_set_channels.
Patch 4: Forbid decreasing the number of active queues if a deactivated
         queue has an AF_XDP socket.
Patch 5: Simplify xdp_clear_umem_at_qid now when ethtool cannot deactivate
         the queue id we are running on.
====================

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

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