Re: [PATCH v2 1/5] tap: Pad non-batched frames to 802.3 minimum (60 bytes) if needed

On Fri, Dec 05, 2025 at 01:51:53AM +0100, Stefano Brivio wrote:

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 Reviewed-by: Laurent Vivier

Reviewed-by: David Gibson

--- tap.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/tap.c b/tap.c index 44b0644..e3ea61c 100644 --- a/tap.c +++ b/tap.c @@ -130,9 +130,18 @@ 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; + uint32_t vnet_len; + + if (l2len < ETH_ZLEN) { + memcpy(padded, data, l2len); + data = padded; + l2len = ETH_ZLEN; + } + + vnet_len = htonl(l2len);

switch (c->mode) { case MODE_PASST: -- 2.43.0

-- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson