Re: [PATCH v2 5/5] tcp, udp: Pad batched frames for vhost-user modes to 60 bytes (802.3 minimum)

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

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On Fri, Dec 05, 2025 at 01:51:57AM +0100, 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 <sbrivio@redhat.com>
> ---
>  tcp.c          |  2 --
>  tcp_internal.h |  1 +
>  tcp_vu.c       | 32 ++++++++++++++++++++++++++++----
>  udp_vu.c       | 13 ++++++++++++-
>  4 files changed, 41 insertions(+), 7 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index c5486bc..8cd062f 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -341,8 +341,6 @@ enum {
>  };
>  #endif
>  
> -/* MSS rounding: see SET_MSS() */
> -#define MSS_DEFAULT			536
>  #define WINDOW_DEFAULT			14600		/* RFC 6928 */
>  
>  #define RTO_INIT			1		/* s, RFC 6298 */
> 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..638813c 100644
> --- a/tcp_vu.c
> +++ b/tcp_vu.c
> @@ -60,6 +60,26 @@ 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;
> +
> +	memset((char *)iov[cnt - 1].iov_base + iov[cnt - 1].iov_len, 0, pad);
> +	iov[cnt - 1].iov_len += pad;

This assumes there's enough allocated space in last buffer to extend
it this way.  I'm pretty sure that's true in practice, but it's not
super obvious from right here.

I can't see a way to avoid that without doing a substantial rework of
both paths.  But we should at least document the assumption in a
comment.

Also, there's not anything really TCP specific about this, so we
should be able to re-use it for UDP, no?

> +}
> +
>  /**
>   * tcp_vu_send_flag() - Send segment with flags to vhost-user (no payload)
>   * @c:		Execution context
> @@ -91,12 +111,11 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
>  	vu_set_element(&flags_elem[0], NULL, &flags_iov[0]);
>  
>  	elem_cnt = vu_collect(vdev, vq, &flags_elem[0], 1,
> -			      hdrlen + sizeof(struct tcp_syn_opts), NULL);
> +			      MAX(hdrlen + sizeof(*opts), ETH_ZLEN), NULL);
>  	if (elem_cnt != 1)
>  		return -1;
>  
> -	ASSERT(flags_elem[0].in_sg[0].iov_len >=
> -	       hdrlen + sizeof(struct tcp_syn_opts));
> +	ASSERT(flags_elem[0].in_sg[0].iov_len >= hdrlen + sizeof(*opts));
>  
>  	vu_set_vnethdr(vdev, flags_elem[0].in_sg[0].iov_base, 1);
>  
> @@ -138,6 +157,8 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)

A few lines above here is where we truncate iov_len[] to match the
actual length of the frame.  Replace that with a call to the
pad/truncate function.

>  	tcp_fill_headers(c, conn, NULL, eh, ip4h, ip6h, th, &payload,
>  			 NULL, seq, !*c->pcap);
>  
> +	tcp_vu_pad(&flags_elem[0].in_sg[0], 1);
> +
>  	if (*c->pcap) {
>  		pcap_iov(&flags_elem[0].in_sg[0], 1,
>  			 sizeof(struct virtio_net_hdr_mrg_rxbuf));
> @@ -211,7 +232,8 @@ static ssize_t tcp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq,
>  
>  		cnt = vu_collect(vdev, vq, &elem[elem_cnt],
>  				 VIRTQUEUE_MAX_SIZE - elem_cnt,
> -				 MIN(mss, fillsize) + hdrlen, &frame_size);
> +				 MAX(MIN(mss, fillsize) + hdrlen, ETH_ZLEN),
> +				 &frame_size);
>  		if (cnt == 0)
>  			break;
>  
> @@ -456,6 +478,8 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
>  
>  		tcp_vu_prepare(c, conn, iov, buf_cnt, &check, !*c->pcap, push);
>  
> +		tcp_vu_pad(iov, buf_cnt);
> +
>  		if (*c->pcap) {
>  			pcap_iov(iov, buf_cnt,
>  				 sizeof(struct virtio_net_hdr_mrg_rxbuf));
> diff --git a/udp_vu.c b/udp_vu.c
> index 099677f..33dbb9a 100644
> --- a/udp_vu.c
> +++ b/udp_vu.c
> @@ -72,8 +72,8 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
>  {
>  	const struct vu_dev *vdev = c->vdev;
>  	int iov_cnt, idx, iov_used;
> +	size_t off, hdrlen, l2len;
>  	struct msghdr msg  = { 0 };
> -	size_t off, hdrlen;
>  
>  	ASSERT(!c->no_udp);
>  
> @@ -116,6 +116,17 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
>  		iov_vu[idx].iov_len = off;
>  	iov_used = idx + !!off;
>  
> +	/* pad 802.3 frame to 60 bytes if needed */
> +	l2len = *dlen + hdrlen - sizeof(struct virtio_net_hdr_mrg_rxbuf);
> +	if (l2len < ETH_ZLEN) {
> +		size_t pad = ETH_ZLEN - l2len;
> +
> +		memset((char *)iov_vu[idx].iov_base + iov_vu[idx].iov_len,
> +		       0, pad);
> +
> +		iov_vu[idx].iov_len += pad;
> +	}
> +
>  	vu_set_vnethdr(vdev, iov_vu[0].iov_base, iov_used);
>  
>  	/* release unused buffers */
> -- 
> 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

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