[PATCH] tcp_vu: Compute IPv4 header checksum if dlen changes

[Laurent Vivier <lvivier@redhat.com>]

In tcp_vu_data_from_sock() we compute IPv4 header checksum only
for the first and the last packets, and re-use the first packet checksum
for all the other packets as the content of the header doesn't change.

It's more accurate to check the dlen value to know if the checksum
should change as dlen is the only information that can change in the
loop.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 tcp_vu.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/tcp_vu.c b/tcp_vu.c
index bbae918c3bb3..4e4e916ded78 100644
--- a/tcp_vu.c
+++ b/tcp_vu.c
@@ -381,12 +381,12 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
 	struct vu_dev *vdev = c->vdev;
 	struct vu_virtq *vq = &vdev->vq[VHOST_USER_RX_QUEUE];
 	const struct flowside *tapside = TAPFLOW(conn);
+	ssize_t len, previous_dlen;
 	size_t fillsize, hdrlen;
 	int v6 = CONN_V6(conn);
 	uint32_t already_sent;
 	const uint16_t *check;
 	int i, iov_cnt;
-	ssize_t len;
 
 	if (!vu_queue_enabled(vq) || !vu_queue_started(vq)) {
 		debug("Got packet, but RX virtqueue not usable yet");
@@ -460,19 +460,17 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
 	 */
 
 	hdrlen = tcp_vu_hdrlen(v6);
-	for (i = 0, check = NULL; i < head_cnt; i++) {
+	for (i = 0, previous_dlen = -1, check = NULL; i < head_cnt; i++) {
 		struct iovec *iov = &elem[head[i]].in_sg[0];
 		int buf_cnt = head[i + 1] - head[i];
 		ssize_t dlen = iov_size(iov, buf_cnt) - hdrlen;
 
 		vu_set_vnethdr(vdev, iov->iov_base, buf_cnt);
 
-		/* we compute IPv4 header checksum only for the
-		 * first and the last, all other checksums are the
-		 * same as the first one
-		 */
-		if (i + 1 == head_cnt)
+		/* The IPv4 header checksum varies only with dlen */
+		if (previous_dlen != dlen)
 			check = NULL;
+		previous_dlen = dlen;
 
 		tcp_vu_prepare(c, conn, iov->iov_base, dlen, &check);
 
-- 
@@ -381,12 +381,12 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
 	struct vu_dev *vdev = c->vdev;
 	struct vu_virtq *vq = &vdev->vq[VHOST_USER_RX_QUEUE];
 	const struct flowside *tapside = TAPFLOW(conn);
+	ssize_t len, previous_dlen;
 	size_t fillsize, hdrlen;
 	int v6 = CONN_V6(conn);
 	uint32_t already_sent;
 	const uint16_t *check;
 	int i, iov_cnt;
-	ssize_t len;
 
 	if (!vu_queue_enabled(vq) || !vu_queue_started(vq)) {
 		debug("Got packet, but RX virtqueue not usable yet");
@@ -460,19 +460,17 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
 	 */
 
 	hdrlen = tcp_vu_hdrlen(v6);
-	for (i = 0, check = NULL; i < head_cnt; i++) {
+	for (i = 0, previous_dlen = -1, check = NULL; i < head_cnt; i++) {
 		struct iovec *iov = &elem[head[i]].in_sg[0];
 		int buf_cnt = head[i + 1] - head[i];
 		ssize_t dlen = iov_size(iov, buf_cnt) - hdrlen;
 
 		vu_set_vnethdr(vdev, iov->iov_base, buf_cnt);
 
-		/* we compute IPv4 header checksum only for the
-		 * first and the last, all other checksums are the
-		 * same as the first one
-		 */
-		if (i + 1 == head_cnt)
+		/* The IPv4 header checksum varies only with dlen */
+		if (previous_dlen != dlen)
 			check = NULL;
+		previous_dlen = dlen;
 
 		tcp_vu_prepare(c, conn, iov->iov_base, dlen, &check);
 
-- 
2.47.0