Re: [PATCH v2 10/10] vhost-user: Centralise Ethernet frame padding in vu_collect() and vu_pad()

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Apr 03, 2026 at 06:38:11PM +0200, Laurent Vivier wrote:
> The previous per-protocol padding done by vu_pad() in tcp_vu.c and
> udp_vu.c was only correct for single-buffer frames: it assumed the
> padding area always fell within the first iov, writing past its end
> with a plain memset().
> 
> It also required each caller to compute MAX(..., ETH_ZLEN + VNET_HLEN)
> for vu_collect() and to call vu_pad() at the right point, duplicating
> the minimum-size logic across protocols.
> 
> Move the Ethernet minimum size enforcement into vu_collect() itself, so
> that enough buffer space is always reserved for padding regardless of
> the requested frame size.
> 
> Rewrite vu_pad() to take a full iovec array and use iov_memset(),
> making it safe for multi-buffer (mergeable rx buffer) frames.
> 
> In tcp_vu_sock_recv(), replace iov_truncate() with iov_skip_bytes():
> now that all consumers receive explicit data lengths, truncating the
> iovecs is no longer needed.  In tcp_vu_data_from_sock(), cap each
> frame's data length against the remaining bytes actually received from
> the socket, so that the last partial frame gets correct headers and
> sequence number advancement.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  iov.c       |  1 -
>  tcp_vu.c    | 34 ++++++++++++++++++----------------
>  udp_vu.c    | 14 ++++++++------
>  vu_common.c | 31 +++++++++++++++----------------
>  vu_common.h |  2 +-
>  5 files changed, 42 insertions(+), 40 deletions(-)
> 
> diff --git a/iov.c b/iov.c
> index dabc4f1ceea3..28c6d40d2986 100644
> --- a/iov.c
> +++ b/iov.c
> @@ -180,7 +180,6 @@ size_t iov_truncate(struct iovec *iov, size_t iov_cnt, size_t size)
>   * 		Will write less than @length bytes if it runs out of space in
>   * 		the iov
>   */
> -/* cppcheck-suppress unusedFunction */
>  void iov_memset(const struct iovec *iov, size_t iov_cnt, size_t offset, int c,
>  		size_t length)
>  {
> diff --git a/tcp_vu.c b/tcp_vu.c
> index 8c1894dca7fe..2dfe14485eee 100644
> --- a/tcp_vu.c
> +++ b/tcp_vu.c
> @@ -88,7 +88,7 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
>  
>  	elem_cnt = vu_collect(vdev, vq, &flags_elem[0], 1,
>  			      &flags_iov[0], 1, NULL,
> -			      MAX(hdrlen + sizeof(*opts), ETH_ZLEN + VNET_HLEN), NULL);
> +			      hdrlen + sizeof(*opts), NULL);
>  	if (elem_cnt != 1)
>  		return -1;
>  
> @@ -128,8 +128,6 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
>  		return ret;
>  	}
>  
> -	l2len = hdrlen + optlen - VNET_HLEN;
> -	iov_truncate(&flags_iov[0], 1, l2len + VNET_HLEN);
>  	payload = IOV_TAIL(flags_elem[0].in_sg, 1, hdrlen);
>  
>  	if (flags & KEEPALIVE)
> @@ -138,17 +136,17 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
>  	tcp_fill_headers(c, conn, eh, ip4h, ip6h, th, &payload,
>  			 optlen, NULL, seq, !*c->pcap);
>  
> -	vu_pad(&flags_elem[0].in_sg[0], l2len);
> -
> +	vu_pad(flags_elem[0].in_sg, 1, hdrlen + optlen);

Is there a reason not to fold vu_pad() into vu_flush()?

>  	vu_flush(vdev, vq, flags_elem, 1, hdrlen + optlen);
>  
> +	l2len = hdrlen + optlen - VNET_HLEN;
>  	if (*c->pcap)
>  		pcap_iov(&flags_elem[0].in_sg[0], 1, VNET_HLEN, l2len);
>  
>  	if (flags & DUP_ACK) {
>  		elem_cnt = vu_collect(vdev, vq, &flags_elem[1], 1,
>  				      &flags_iov[1], 1, NULL,
> -				      flags_elem[0].in_sg[0].iov_len, NULL);
> +				      hdrlen + optlen, NULL);
>  		if (elem_cnt == 1 &&
>  		    flags_elem[1].in_sg[0].iov_len >=
>  		    flags_elem[0].in_sg[0].iov_len) {
> @@ -213,7 +211,7 @@ static ssize_t tcp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq,
>  				 ARRAY_SIZE(elem) - elem_cnt,
>  				 &iov_vu[DISCARD_IOV_NUM + iov_used],
>  				 VIRTQUEUE_MAX_SIZE - iov_used, &in_total,
> -				 MAX(MIN(mss, fillsize) + hdrlen, ETH_ZLEN + VNET_HLEN),
> +				 MIN(mss, fillsize) + hdrlen,
>  				 &frame_size);
>  		if (cnt == 0)
>  			break;
> @@ -249,8 +247,11 @@ static ssize_t tcp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq,
>  	if (!peek_offset_cap)
>  		ret -= already_sent;
>  
> -	/* adjust iov number and length of the last iov */
> -	i = iov_truncate(&iov_vu[DISCARD_IOV_NUM], iov_used, ret);
> +	i = iov_skip_bytes(&iov_vu[DISCARD_IOV_NUM], iov_used,
> +			   MAX(hdrlen + ret, VNET_HLEN + ETH_ZLEN),
> +			   NULL);
> +	if ((size_t)i < iov_used)
> +		i++;
>  
>  	/* adjust head count */
>  	while (*head_cnt > 0 && head[*head_cnt - 1] >= i)
> @@ -447,11 +448,13 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
>  		size_t frame_size = iov_size(iov, buf_cnt);
>  		bool push = i == head_cnt - 1;
>  		ssize_t dlen;
> -		size_t l2len;
>  
>  		assert(frame_size >= hdrlen);
>  
>  		dlen = frame_size - hdrlen;
> +		if (dlen > len)
> +			dlen = len;
> +		len -= dlen;
>  
>  		/* The IPv4 header checksum varies only with dlen */
>  		if (previous_dlen != dlen)
> @@ -460,14 +463,13 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
>  
>  		tcp_vu_prepare(c, conn, iov, buf_cnt, dlen, &check, !*c->pcap, push);
>  
> -		/* Pad first/single buffer only, it's at least ETH_ZLEN long */
> -		l2len = dlen + hdrlen - VNET_HLEN;
> -		vu_pad(iov, l2len);
> -
> +		vu_pad(elem[head[i]].in_sg, buf_cnt, dlen + hdrlen);
>  		vu_flush(vdev, vq, &elem[head[i]], buf_cnt, dlen + hdrlen);
>  
> -		if (*c->pcap)
> -			pcap_iov(iov, buf_cnt, VNET_HLEN, l2len);
> +		if (*c->pcap) {
> +			pcap_iov(iov, buf_cnt, VNET_HLEN,
> +				 dlen + hdrlen - VNET_HLEN);
> +		}
>  
>  		conn->seq_to_tap += dlen;
>  	}
> diff --git a/udp_vu.c b/udp_vu.c
> index 4641f42eb5c4..30af64034516 100644
> --- a/udp_vu.c
> +++ b/udp_vu.c
> @@ -65,7 +65,7 @@ static size_t udp_vu_hdrlen(bool v6)
>  static ssize_t udp_vu_sock_recv(struct iovec *iov, size_t *cnt, int s, bool v6)
>  {
>  	struct msghdr msg  = { 0 };
> -	size_t hdrlen, l2len;
> +	size_t hdrlen, iov_used;
>  	ssize_t dlen;
>  
>  	/* compute L2 header length */
> @@ -88,11 +88,12 @@ static ssize_t udp_vu_sock_recv(struct iovec *iov, size_t *cnt, int s, bool v6)
>  	iov[0].iov_base = (char *)iov[0].iov_base - hdrlen;
>  	iov[0].iov_len += hdrlen;
>  
> -	*cnt = iov_truncate(iov, *cnt, dlen + hdrlen);
> -
> -	/* pad frame to 60 bytes: first buffer is at least ETH_ZLEN long */
> -	l2len = dlen + hdrlen - VNET_HLEN;
> -	vu_pad(&iov[0], l2len);
> +	iov_used = iov_skip_bytes(iov, *cnt,
> +				  MAX(dlen + hdrlen, VNET_HLEN + ETH_ZLEN),
> +				  NULL);
> +	if (iov_used < *cnt)
> +		iov_used++;
> +	*cnt = iov_used; /* one iovec per element */
>  
>  	return dlen;
>  }
> @@ -234,6 +235,7 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
>  				pcap_iov(iov_vu, iov_cnt, VNET_HLEN,
>  					 hdrlen + dlen - VNET_HLEN);
>  			}
> +			vu_pad(iov_vu, iov_cnt, hdrlen + dlen);
>  			vu_flush(vdev, vq, elem, elem_used, hdrlen + dlen);
>  			vu_queue_notify(vdev, vq);
>  		}
> diff --git a/vu_common.c b/vu_common.c
> index 704e908aa02c..d07f584f228a 100644
> --- a/vu_common.c
> +++ b/vu_common.c
> @@ -74,6 +74,7 @@ int vu_collect(const struct vu_dev *vdev, struct vu_virtq *vq,
>  	size_t current_iov = 0;
>  	int elem_cnt = 0;
>  
> +	size = MAX(size, ETH_ZLEN /* Ethernet minimum size */ + VNET_HLEN);
>  	while (current_size < size && elem_cnt < max_elem &&
>  	       current_iov < max_in_sg) {
>  		int ret;
> @@ -261,29 +262,27 @@ int vu_send_single(const struct ctx *c, const void *buf, size_t size)
>  		return -1;
>  	}
>  
> -	size += VNET_HLEN;
>  	elem_cnt = vu_collect(vdev, vq, elem, ARRAY_SIZE(elem), in_sg,
> -			      ARRAY_SIZE(in_sg), &in_total, size, &total);
> -	if (elem_cnt == 0 || total < size) {
> +			      ARRAY_SIZE(in_sg), &in_total, VNET_HLEN + size, &total);
> +	if (elem_cnt == 0 || total < VNET_HLEN + size) {
>  		debug("vu_send_single: no space to send the data "
>  		      "elem_cnt %d size %zu", elem_cnt, total);
>  		goto err;
>  	}
>  
> -	total -= VNET_HLEN;
> -
>  	/* copy data from the buffer to the iovec */
> -	iov_from_buf(in_sg, in_total, VNET_HLEN, buf, total);
> +	iov_from_buf(in_sg, in_total, VNET_HLEN, buf, size);
>  
>  	if (*c->pcap)
>  		pcap_iov(in_sg, in_total, VNET_HLEN, size);
>  
> +	vu_pad(in_sg, in_total, VNET_HLEN + size);
>  	vu_flush(vdev, vq, elem, elem_cnt, VNET_HLEN + size);
>  	vu_queue_notify(vdev, vq);
>  
> -	trace("vhost-user sent %zu", total);
> +	trace("vhost-user sent %zu", size);
>  
> -	return total;
> +	return size;
>  err:
>  	for (i = 0; i < elem_cnt; i++)
>  		vu_queue_detach_element(vq);
> @@ -292,15 +291,15 @@ err:
>  }
>  
>  /**
> - * vu_pad() - Pad 802.3 frame to minimum length (60 bytes) if needed
> - * @iov:	Buffer in iovec array where end of 802.3 frame is stored
> - * @l2len:	Layer-2 length already filled in frame
> + * vu_pad() - Pad short frames to minimum Ethernet length and truncate iovec
> + * @iov:	Pointer to iovec array
> + * @cnt:	Number of entries in @iov
> + * @frame_len:	Data length in @iov (including virtio-net header)
>   */
> -void vu_pad(struct iovec *iov, size_t l2len)
> +void vu_pad(const struct iovec *iov, size_t cnt, size_t frame_len)
>  {
> -	if (l2len >= ETH_ZLEN)
> -		return;
> +	size_t min_frame_len = ETH_ZLEN + VNET_HLEN;
>  
> -	memset((char *)iov->iov_base + iov->iov_len, 0, ETH_ZLEN - l2len);
> -	iov->iov_len += ETH_ZLEN - l2len;
> +	if (frame_len < min_frame_len)
> +		iov_memset(iov, cnt, frame_len, 0, min_frame_len - frame_len);
>  }
> diff --git a/vu_common.h b/vu_common.h
> index 77d1849e6115..51f70084a7cb 100644
> --- a/vu_common.h
> +++ b/vu_common.h
> @@ -44,6 +44,6 @@ void vu_flush(const struct vu_dev *vdev, struct vu_virtq *vq,
>  void vu_kick_cb(struct vu_dev *vdev, union epoll_ref ref,
>  		const struct timespec *now);
>  int vu_send_single(const struct ctx *c, const void *buf, size_t size);
> -void vu_pad(struct iovec *iov, size_t l2len);
> +void vu_pad(const struct iovec *iov, size_t cnt, size_t frame_len);
>  
>  #endif /* VU_COMMON_H */
> -- 
> 2.53.0
> 

-- 
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

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