ibmvnic: Reduce memcpys in tx descriptor generation

Previously when creating the header descriptors, the driver would:
1. allocate a temporary buffer on the stack (in build_hdr_descs_arr)
2. memcpy the header info into the temporary buffer (in build_hdr_data)
3. memcpy the temp buffer into a local variable (in create_hdr_descs)
4. copy the local variable into the return buffer (in create_hdr_descs)

Since, there is no opportunity for errors during this process, the temp
buffer is not needed and work can be done on the return buffer directly.

Repurpose build_hdr_data() to only calculate the header lengths. Rename
it to get_hdr_lens().
Edit create_hdr_descs() to read from the skb directly and copy directly
into the returned useful buffer.

The process now involves less memory and write operations while
also being more readable.

Signed-off-by: Nick Child <nnac123@linux.ibm.com>
Link: https://patch.msgid.link/20240807211809.1259563-4-nnac123@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/ibm/ibmvnic.c

@@ -2140,56 +2140,49 @@ static int ibmvnic_close(struct net_device *netdev)
 }
 
 /**
- * build_hdr_data - creates L2/L3/L4 header data buffer
+ * get_hdr_lens - fills list of L2/L3/L4 hdr lens
  * @hdr_field: bitfield determining needed headers
  * @skb: socket buffer
- * @hdr_len: array of header lengths
- * @hdr_data: buffer to write the header to
+ * @hdr_len: array of header lengths to be filled
  *
  * Reads hdr_field to determine which headers are needed by firmware.
  * Builds a buffer containing these headers.  Saves individual header
  * lengths and total buffer length to be used to build descriptors.
+ *
+ * Return: total len of all headers
  */
-static int build_hdr_data(u8 hdr_field, struct sk_buff *skb,
-			  int *hdr_len, u8 *hdr_data)
+static int get_hdr_lens(u8 hdr_field, struct sk_buff *skb,
+			int *hdr_len)
 {
 	int len = 0;
-	u8 *hdr;
-
 
-	if (skb->protocol == htons(ETH_P_IP)) {
-		if (ip_hdr(skb)->protocol == IPPROTO_TCP)
-			hdr_len[2] = tcp_hdrlen(skb);
-		else if (ip_hdr(skb)->protocol == IPPROTO_UDP)
-			hdr_len[2] = sizeof(struct udphdr);
-	} else if (skb->protocol == htons(ETH_P_IPV6)) {
-		if (ipv6_hdr(skb)->nexthdr == IPPROTO_TCP)
-			hdr_len[2] = tcp_hdrlen(skb);
-		else if (ipv6_hdr(skb)->nexthdr == IPPROTO_UDP)
-			hdr_len[2] = sizeof(struct udphdr);
-	}
 
 	if ((hdr_field >> 6) & 1) {
 		hdr_len[0] = skb_mac_header_len(skb);
-		hdr = skb_mac_header(skb);
-		memcpy(hdr_data, hdr, hdr_len[0]);
 		len += hdr_len[0];
 	}
 
 	if ((hdr_field >> 5) & 1) {
 		hdr_len[1] = skb_network_header_len(skb);
-		hdr = skb_network_header(skb);
-		memcpy(hdr_data + len, hdr, hdr_len[1]);
 		len += hdr_len[1];
 	}
 
-	if ((hdr_field >> 4) & 1) {
-		hdr = skb_transport_header(skb);
-		memcpy(hdr_data + len, hdr, hdr_len[2]);
-		len += hdr_len[2];
+	if (!((hdr_field >> 4) & 1))
+		return len;
+
+	if (skb->protocol == htons(ETH_P_IP)) {
+		if (ip_hdr(skb)->protocol == IPPROTO_TCP)
+			hdr_len[2] = tcp_hdrlen(skb);
+		else if (ip_hdr(skb)->protocol == IPPROTO_UDP)
+			hdr_len[2] = sizeof(struct udphdr);
+	} else if (skb->protocol == htons(ETH_P_IPV6)) {
+		if (ipv6_hdr(skb)->nexthdr == IPPROTO_TCP)
+			hdr_len[2] = tcp_hdrlen(skb);
+		else if (ipv6_hdr(skb)->nexthdr == IPPROTO_UDP)
+			hdr_len[2] = sizeof(struct udphdr);
 	}
 
-	return len;
+	return len + hdr_len[2];
 }
 
 /**
@@ -2202,12 +2195,14 @@ static int build_hdr_data(u8 hdr_field, struct sk_buff *skb,
  *
  * Creates header and, if needed, header extension descriptors and
  * places them in a descriptor array, scrq_arr
+ *
+ * Return: Number of header descs
  */
 
 static int create_hdr_descs(u8 hdr_field, u8 *hdr_data, int len, int *hdr_len,
 			    union sub_crq *scrq_arr)
 {
-	union sub_crq hdr_desc;
+	union sub_crq *hdr_desc;
 	int tmp_len = len;
 	int num_descs = 0;
 	u8 *data, *cur;
@@ -2216,28 +2211,26 @@ static int create_hdr_descs(u8 hdr_field, u8 *hdr_data, int len, int *hdr_len,
 	while (tmp_len > 0) {
 		cur = hdr_data + len - tmp_len;
 
-		memset(&hdr_desc, 0, sizeof(hdr_desc));
-		if (cur != hdr_data) {
-			data = hdr_desc.hdr_ext.data;
+		hdr_desc = &scrq_arr[num_descs];
+		if (num_descs) {
+			data = hdr_desc->hdr_ext.data;
 			tmp = tmp_len > 29 ? 29 : tmp_len;
-			hdr_desc.hdr_ext.first = IBMVNIC_CRQ_CMD;
-			hdr_desc.hdr_ext.type = IBMVNIC_HDR_EXT_DESC;
-			hdr_desc.hdr_ext.len = tmp;
+			hdr_desc->hdr_ext.first = IBMVNIC_CRQ_CMD;
+			hdr_desc->hdr_ext.type = IBMVNIC_HDR_EXT_DESC;
+			hdr_desc->hdr_ext.len = tmp;
 		} else {
-			data = hdr_desc.hdr.data;
+			data = hdr_desc->hdr.data;
 			tmp = tmp_len > 24 ? 24 : tmp_len;
-			hdr_desc.hdr.first = IBMVNIC_CRQ_CMD;
-			hdr_desc.hdr.type = IBMVNIC_HDR_DESC;
-			hdr_desc.hdr.len = tmp;
-			hdr_desc.hdr.l2_len = (u8)hdr_len[0];
-			hdr_desc.hdr.l3_len = cpu_to_be16((u16)hdr_len[1]);
-			hdr_desc.hdr.l4_len = (u8)hdr_len[2];
-			hdr_desc.hdr.flag = hdr_field << 1;
+			hdr_desc->hdr.first = IBMVNIC_CRQ_CMD;
+			hdr_desc->hdr.type = IBMVNIC_HDR_DESC;
+			hdr_desc->hdr.len = tmp;
+			hdr_desc->hdr.l2_len = (u8)hdr_len[0];
+			hdr_desc->hdr.l3_len = cpu_to_be16((u16)hdr_len[1]);
+			hdr_desc->hdr.l4_len = (u8)hdr_len[2];
+			hdr_desc->hdr.flag = hdr_field << 1;
 		}
 		memcpy(data, cur, tmp);
 		tmp_len -= tmp;
-		*scrq_arr = hdr_desc;
-		scrq_arr++;
 		num_descs++;
 	}
 
@@ -2260,13 +2253,11 @@ static void build_hdr_descs_arr(struct sk_buff *skb,
 				int *num_entries, u8 hdr_field)
 {
 	int hdr_len[3] = {0, 0, 0};
-	u8 hdr_data[140] = {0};
 	int tot_len;
 
-	tot_len = build_hdr_data(hdr_field, skb, hdr_len,
-				 hdr_data);
-	*num_entries += create_hdr_descs(hdr_field, hdr_data, tot_len, hdr_len,
-					 indir_arr + 1);
+	tot_len = get_hdr_lens(hdr_field, skb, hdr_len);
+	*num_entries += create_hdr_descs(hdr_field, skb_mac_header(skb),
+					 tot_len, hdr_len, indir_arr + 1);
 }
 
 static int ibmvnic_xmit_workarounds(struct sk_buff *skb,