The RX queue has an array of `gve_rx_buf_state_dqo` objects. All
allocated pages have an associated buf_state object. When a buffer is
posted on the RX buffer queue, the buffer ID will be the buf_state's
index into the RX queue's array.

On packet reception, the RX queue will have one descriptor for each
buffer associated with a received packet. Each RX descriptor will have
a buffer_id that was posted on the buffer queue.

Notable mentions:

- We use a default buffer size of 2048 bytes. Based on page size, we
  may post separate sections of a single page as separate buffers.

- The driver holds an extra reference on pages passed up the receive
  path with an skb and keeps these pages on a list. When posting new
  buffers to the NIC, we check if any of these pages has only our
  reference, or another buffer sized segment of the page has no
  references. If so, it is free to reuse. This page recycling approach
  is a common netdev optimization that reduces page alloc/free calls.

- Pages in the free list have a page_count bias in order to avoid an
  atomic increment of pagecount every time we attempt to reuse a page.
  # references = page_count() - bias

- In order to track when a page is safe to reuse, we keep track of the
  last offset which had a single SKB reference. When this occurs, it
  implies that every single other offset is reusable. Otherwise, we
  don't know if offsets can be safely reused.

- We maintain two free lists of pages. List #1 (recycled_buf_states)
  contains pages we know can be reused right away. List #2
  (used_buf_states) contains pages which cannot be used right away. We
  only attempt to get pages from list #2 when list #1 is empty. We only
  attempt to use a small fixed number pages from list #2 before giving
  up and allocating a new page. Both lists are FIFOs in hope that by the
  time we attempt to reuse a page, the references were dropped.
Signed-off-by: Bailey Forrest <bcf@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Catherine Sullivan <csully@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>