Re: [RFC PATCH 5/5] tcp, udp: Pad batched frames for vhost-user modes to 60 bytes (802.3 minimum)

4 Nov 2025

On Tue, 4 Nov 2025 17:28:52 +0100
Laurent Vivier  wrote:
...
On 11/4/25 17:09, Stefano Brivio wrote:
...
On Tue, 4 Nov 2025 16:50:43 +0100
Laurent Vivier  wrote:
...
On 11/3/25 11:16, Stefano Brivio wrote:
...
For both TCP and UDP, we request vhost-user buffers that are large
enough to reach ETH_ZLEN (60 bytes), so padding is just a matter of
increasing the appropriate iov_len and clearing bytes in the buffer
as needed.
Link: https://bugs.passt.top/show_bug.cgi?id=166
Signed-off-by: Stefano Brivio 
---
   tcp.c          |  2 --
   tcp_internal.h |  1 +
   tcp_vu.c       | 27 +++++++++++++++++++++++++++
   udp_vu.c       | 11 ++++++++++-
   4 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/tcp.c b/tcp.c
index e91c0cf..039688d 100644
--- a/tcp.c
+++ b/tcp.c
@@ -335,8 +335,6 @@ enum {
   };
   #endif
-/* MSS rounding: see SET_MSS() */
-#define MSS_DEFAULT			536
   #define WINDOW_DEFAULT			14600		/* RFC 6928 */
#define ACK_INTERVAL			10		/* ms */
diff --git a/tcp_internal.h b/tcp_internal.h
index 5f8fb35..d2295c9 100644
--- a/tcp_internal.h
+++ b/tcp_internal.h
@@ -12,6 +12,7 @@
   #define BUF_DISCARD_SIZE	(1 << 20)
   #define DISCARD_IOV_NUM		DIV_ROUND_UP(MAX_WINDOW, BUF_DISCARD_SIZE)
+#define MSS_DEFAULT /* and minimum */	536 /* as it comes from minimum MTU */
   #define MSS4				ROUND_DOWN(IP_MAX_MTU -		   \
       					   sizeof(struct tcphdr) - \
       					   sizeof(struct iphdr),   \
diff --git a/tcp_vu.c b/tcp_vu.c
index 1c81ce3..7239401 100644
--- a/tcp_vu.c
+++ b/tcp_vu.c
@@ -60,6 +60,29 @@ static size_t tcp_vu_hdrlen(bool v6)
       return hdrlen;
   }
+/**
+ * tcp_vu_pad() - Pad 802.3 frame to minimum length (60 bytes) if needed
+ * @iov:	iovec array storing 802.3 frame with TCP segment inside
+ * @cnt:	Number of entries in @iov
+ */
+static void tcp_vu_pad(struct iovec *iov, size_t cnt)
+{
+	size_t l2len, pad;
+
+	ASSERT(iov_size(iov, cnt) >= sizeof(struct virtio_net_hdr_mrg_rxbuf));
+	l2len = iov_size(iov, cnt) - sizeof(struct virtio_net_hdr_mrg_rxbuf);
+	if (l2len >= ETH_ZLEN)
+		return;
+
+	pad = ETH_ZLEN - l2len;
+
+	/* tcp_vu_sock_recv() requests at least MSS-sized vhost-user buffers */
+	static_assert(ETH_ZLEN <= MSS_DEFAULT);
+
+	memset(&iov[cnt - 1].iov_base + iov[cnt - 1].iov_len, 0, pad);
I think it should be
memset((char *)iov[cnt - 1].iov_base + iov[cnt - 1].iov_len, 0, pad);
Right, thanks, I always forget that sizeof(void) being 1 is a gcc
extension:
https://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html
What's rather confusing, actually, is that even if I explicitly enable
-Wpointer-arith, I don't get a warning for that. Any clue?
in fact it's sizeof(void **) as iov[cnt - 1].iov_base is (void *) and you take &(void *).
Normally gcc spits out warnings when we do .iov_base + .iov_len, but as you take address 
of iov_base all is fine :P
Ouch, wow, how does it even work? I checked captures of most functional
tests with vhost-user, connections worked and the right ACK segments
had the right amount of padding.

I guess it's just that there was nothing fundamental at the resulting
address and we already happened to have zeroes in the buffers.

Thanks for spotting that, I'll fix in v2.

-- 
Stefano

    