gve: Fix an edge case for TSO skb validity check

The NIC requires each TSO segment to not span more than 10
descriptors. NIC further requires each descriptor to not exceed
16KB - 1 (GVE_TX_MAX_BUF_SIZE_DQO).

The descriptors for an skb are generated by
gve_tx_add_skb_no_copy_dqo() for DQO RDA queue format.
gve_tx_add_skb_no_copy_dqo() loops through each skb frag and
generates a descriptor for the entire frag if the frag size is
not greater than GVE_TX_MAX_BUF_SIZE_DQO. If the frag size is
greater than GVE_TX_MAX_BUF_SIZE_DQO, it is split into descriptor(s)
of size GVE_TX_MAX_BUF_SIZE_DQO and a descriptor is generated for
the remainder (frag size % GVE_TX_MAX_BUF_SIZE_DQO).

gve_can_send_tso() checks if the descriptors thus generated for an
skb would meet the requirement that each TSO-segment not span more
than 10 descriptors. However, the current code misses an edge case
when a TSO segment spans multiple descriptors within a large frag.
This change fixes the edge case.

gve_can_send_tso() relies on the assumption that max gso size (9728)
is less than GVE_TX_MAX_BUF_SIZE_DQO and therefore within an skb
fragment a TSO segment can never span more than 2 descriptors.

Fixes: a57e5de ("gve: DQO: Add TX path")
Signed-off-by: Praveen Kaligineedi <[email protected]>
Signed-off-by: Bailey Forrest <[email protected]>
Reviewed-by: Jeroen de Borst <[email protected]>
Cc: [email protected]
Reviewed-by: Willem de Bruijn <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: baileyforrest

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

@@ -866,22 +866,42 @@ static bool gve_can_send_tso(const struct sk_buff *skb)
 	const int header_len = skb_tcp_all_headers(skb);
 	const int gso_size = shinfo->gso_size;
 	int cur_seg_num_bufs;
+	int prev_frag_size;
 	int cur_seg_size;
 	int i;
 
 	cur_seg_size = skb_headlen(skb) - header_len;
+	prev_frag_size = skb_headlen(skb);
 	cur_seg_num_bufs = cur_seg_size > 0;
 
 	for (i = 0; i < shinfo->nr_frags; i++) {
 		if (cur_seg_size >= gso_size) {
 			cur_seg_size %= gso_size;
 			cur_seg_num_bufs = cur_seg_size > 0;
+
+			if (prev_frag_size > GVE_TX_MAX_BUF_SIZE_DQO) {
+				int prev_frag_remain = prev_frag_size %
+					GVE_TX_MAX_BUF_SIZE_DQO;
+
+				/* If the last descriptor of the previous frag
+				 * is less than cur_seg_size, the segment will
+				 * span two descriptors in the previous frag.
+				 * Since max gso size (9728) is less than
+				 * GVE_TX_MAX_BUF_SIZE_DQO, it is impossible
+				 * for the segment to span more than two
+				 * descriptors.
+				 */
+				if (prev_frag_remain &&
+				    cur_seg_size > prev_frag_remain)
+					cur_seg_num_bufs++;
+			}
 		}
 
 		if (unlikely(++cur_seg_num_bufs > max_bufs_per_seg))
 			return false;
 
-		cur_seg_size += skb_frag_size(&shinfo->frags[i]);
+		prev_frag_size = skb_frag_size(&shinfo->frags[i]);
+		cur_seg_size += prev_frag_size;
 	}
 
 	return true;