gve: Secure enough bytes in the first TX desc for all TCP pkts

Non-GSO TCP packets whose SKBs' linear portion did not include the
entire TCP header were not populating the first Tx descriptor with
as many bytes as the vNIC expected. This change ensures that all
TCP packets populate the first descriptor with the correct number of
bytes.

Fixes: 893ce44d ("gve: Add basic driver framework for Compute Engine Virtual NIC")
Signed-off-by: Shailend Chand <shailend@google.com>
Link: https://lore.kernel.org/r/20230403172809.2939306-1-shailend@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/google/gve/gve.h

@@ -47,6 +47,8 @@
 
 #define GVE_RX_BUFFER_SIZE_DQO 2048
 
+#define GVE_GQ_TX_MIN_PKT_DESC_BYTES 182
+
 /* Each slot in the desc ring has a 1:1 mapping to a slot in the data ring */
 struct gve_rx_desc_queue {
 	struct gve_rx_desc *desc_ring; /* the descriptor ring */

drivers/net/ethernet/google/gve/gve_tx.c

@@ -284,8 +284,8 @@ static inline int gve_skb_fifo_bytes_required(struct gve_tx_ring *tx,
 	int bytes;
 	int hlen;
 
-	hlen = skb_is_gso(skb) ? skb_checksum_start_offset(skb) +
-				 tcp_hdrlen(skb) : skb_headlen(skb);
+	hlen = skb_is_gso(skb) ? skb_checksum_start_offset(skb) + tcp_hdrlen(skb) :
+				 min_t(int, GVE_GQ_TX_MIN_PKT_DESC_BYTES, skb->len);
 
 	pad_bytes = gve_tx_fifo_pad_alloc_one_frag(&tx->tx_fifo,
 						   hlen);
@@ -454,13 +454,11 @@ static int gve_tx_add_skb_copy(struct gve_priv *priv, struct gve_tx_ring *tx, st
 	pkt_desc = &tx->desc[idx];
 
 	l4_hdr_offset = skb_checksum_start_offset(skb);
-	/* If the skb is gso, then we want the tcp header in the first segment
-	 * otherwise we want the linear portion of the skb (which will contain
-	 * the checksum because skb->csum_start and skb->csum_offset are given
-	 * relative to skb->head) in the first segment.
+	/* If the skb is gso, then we want the tcp header alone in the first segment
+	 * otherwise we want the minimum required by the gVNIC spec.
 	 */
 	hlen = is_gso ? l4_hdr_offset + tcp_hdrlen(skb) :
-			skb_headlen(skb);
+			min_t(int, GVE_GQ_TX_MIN_PKT_DESC_BYTES, skb->len);
 
 	info->skb =  skb;
 	/* We don't want to split the header, so if necessary, pad to the end