[RFC PATCH 5/5] tcp, udp: Pad batched frames for vhost-user modes to 60 bytes (802.3 minimum)

[Stefano Brivio <sbrivio@redhat.com>]

For both TCP and UDP, we request vhost-user buffers that are large
enough to reach ETH_ZLEN (60 bytes), so padding is just a matter of
increasing the appropriate iov_len and clearing bytes in the buffer
as needed.

Link: https://bugs.passt.top/show_bug.cgi?id=166
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 tcp.c          |  2 --
 tcp_internal.h |  1 +
 tcp_vu.c       | 27 +++++++++++++++++++++++++++
 udp_vu.c       | 11 ++++++++++-
 4 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/tcp.c b/tcp.c
index e91c0cf..039688d 100644
--- a/tcp.c
+++ b/tcp.c
@@ -335,8 +335,6 @@ enum {
 };
 #endif
 
-/* MSS rounding: see SET_MSS() */
-#define MSS_DEFAULT			536
 #define WINDOW_DEFAULT			14600		/* RFC 6928 */
 
 #define ACK_INTERVAL			10		/* ms */
diff --git a/tcp_internal.h b/tcp_internal.h
index 5f8fb35..d2295c9 100644
--- a/tcp_internal.h
+++ b/tcp_internal.h
@@ -12,6 +12,7 @@
 #define BUF_DISCARD_SIZE	(1 << 20)
 #define DISCARD_IOV_NUM		DIV_ROUND_UP(MAX_WINDOW, BUF_DISCARD_SIZE)
 
+#define MSS_DEFAULT /* and minimum */	536 /* as it comes from minimum MTU */
 #define MSS4				ROUND_DOWN(IP_MAX_MTU -		   \
 						   sizeof(struct tcphdr) - \
 						   sizeof(struct iphdr),   \
diff --git a/tcp_vu.c b/tcp_vu.c
index 1c81ce3..7239401 100644
--- a/tcp_vu.c
+++ b/tcp_vu.c
@@ -60,6 +60,29 @@ static size_t tcp_vu_hdrlen(bool v6)
 	return hdrlen;
 }
 
+/**
+ * tcp_vu_pad() - Pad 802.3 frame to minimum length (60 bytes) if needed
+ * @iov:	iovec array storing 802.3 frame with TCP segment inside
+ * @cnt:	Number of entries in @iov
+ */
+static void tcp_vu_pad(struct iovec *iov, size_t cnt)
+{
+	size_t l2len, pad;
+
+	ASSERT(iov_size(iov, cnt) >= sizeof(struct virtio_net_hdr_mrg_rxbuf));
+	l2len = iov_size(iov, cnt) - sizeof(struct virtio_net_hdr_mrg_rxbuf);
+	if (l2len >= ETH_ZLEN)
+		return;
+
+	pad = ETH_ZLEN - l2len;
+
+	/* tcp_vu_sock_recv() requests at least MSS-sized vhost-user buffers */
+	static_assert(ETH_ZLEN <= MSS_DEFAULT);
+
+	memset(&iov[cnt - 1].iov_base + iov[cnt - 1].iov_len, 0, pad);
+	iov[cnt - 1].iov_len += pad;
+}
+
 /**
  * tcp_vu_send_flag() - Send segment with flags to vhost-user (no payload)
  * @c:		Execution context
@@ -138,6 +161,8 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
 	tcp_fill_headers(c, conn, NULL, eh, ip4h, ip6h, th, &payload,
 			 NULL, seq, !*c->pcap);
 
+	tcp_vu_pad(&flags_elem[0].in_sg[0], 1);
+
 	if (*c->pcap) {
 		pcap_iov(&flags_elem[0].in_sg[0], 1,
 			 sizeof(struct virtio_net_hdr_mrg_rxbuf));
@@ -456,6 +481,8 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
 
 		tcp_vu_prepare(c, conn, iov, buf_cnt, &check, !*c->pcap, push);
 
+		tcp_vu_pad(iov, buf_cnt);
+
 		if (*c->pcap) {
 			pcap_iov(iov, buf_cnt,
 				 sizeof(struct virtio_net_hdr_mrg_rxbuf));
diff --git a/udp_vu.c b/udp_vu.c
index 099677f..1b60860 100644
--- a/udp_vu.c
+++ b/udp_vu.c
@@ -72,8 +72,8 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
 {
 	const struct vu_dev *vdev = c->vdev;
 	int iov_cnt, idx, iov_used;
+	size_t off, hdrlen, l2len;
 	struct msghdr msg  = { 0 };
-	size_t off, hdrlen;
 
 	ASSERT(!c->no_udp);
 
@@ -116,6 +116,15 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
 		iov_vu[idx].iov_len = off;
 	iov_used = idx + !!off;
 
+	/* pad 802.3 frame to 60 bytes if needed */
+	l2len = *dlen + hdrlen - sizeof(struct virtio_net_hdr_mrg_rxbuf);
+	if (l2len < ETH_ZLEN) {
+		size_t pad = ETH_ZLEN - l2len;
+
+		iov_vu[idx].iov_len += pad;
+		memset(&iov_vu[idx].iov_base + off, 0, pad);
+	}
+
 	vu_set_vnethdr(vdev, iov_vu[0].iov_base, iov_used);
 
 	/* release unused buffers */
-- 
2.43.0