From mboxrd@z Thu Jan 1 00:00:00 1970 Received: by passt.top (Postfix, from userid 1000) id 5B90E5A061B; Mon, 03 Nov 2025 11:16:12 +0100 (CET) From: Stefano Brivio To: passt-dev@passt.top, Laurent Vivier Subject: [RFC PATCH 1/5] tap: Pad non-batched frames to 802.3 minimum (60 bytes) if needed Date: Mon, 3 Nov 2025 11:16:08 +0100 Message-ID: <20251103101612.1412079-2-sbrivio@redhat.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20251103101612.1412079-1-sbrivio@redhat.com> References: <20251103101612.1412079-1-sbrivio@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Message-ID-Hash: BGTZ6U3RGJ4A53WAPB2UBTMO55AUTLQD X-Message-ID-Hash: BGTZ6U3RGJ4A53WAPB2UBTMO55AUTLQD X-MailFrom: sbrivio@passt.top X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: David Gibson X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: 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); -- 2.43.0