tap: Pad non-batched frames to 802.3 minimum (60 bytes) if needed

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>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>

0 files changed, 0 insertions, 0 deletions