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

On 11/3/25 11:16, 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 --- 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);

with vnet_len updated: Reviewed-by: Laurent Vivier