[RFC PATCH 1/5] tap: Pad non-batched frames to 802.3 minimum (60 bytes) if needed

[Stefano Brivio <sbrivio@redhat.com>]

IEEE 802.3 requires a minimum frame payload of 46 bytes, without a
802.1Q tag. Add padding for the simple tap_send_single() case using
a zero-filled 60-byte buffer and copying data to it if needed.

In theory, we could add a further element in the iovec array, say:

	uint8_t padding[ETH_ZLEN] = { 0 };
	struct iovec iov[3];

	...

		if (l2len < ETH_ZLEN) {
			iov[iovcnt].iov_base = (void *)padding;
			iov[iovcnt].iov_len = ETH_ZLEN - l2len;
			iovcnt++;
		}

and avoid a copy, but that would substantially complicate the
vhost-user path, and it's questionable whether passing a reference
to a further buffer actually causes lower overhead than the simple
copy.

Link: https://bugs.passt.top/show_bug.cgi?id=166
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 tap.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tap.c b/tap.c
index bb139d6..8d8d84b 100644
--- a/tap.c
+++ b/tap.c
@@ -131,9 +131,16 @@ unsigned long tap_l2_max_len(const struct ctx *c)
 void tap_send_single(const struct ctx *c, const void *data, size_t l2len)
 {
 	uint32_t vnet_len = htonl(l2len);
+	uint8_t padded[ETH_ZLEN] = { 0 };
 	struct iovec iov[2];
 	size_t iovcnt = 0;
 
+	if (l2len < ETH_ZLEN) {
+		memcpy(padded, data, l2len);
+		data = padded;
+		l2len = ETH_ZLEN;
+	}
+
 	switch (c->mode) {
 	case MODE_PASST:
 		iov[iovcnt] = IOV_OF_LVALUE(vnet_len);
-- 
2.43.0