Re: [PATCH v7 4/4] tcp_vu: Support multibuffer frames in tcp_vu_send_flag()

[Laurent Vivier <lvivier@redhat.com>]

On 5/22/26 03:24, David Gibson wrote:
> On Wed, May 20, 2026 at 05:18:27PM +0200, Laurent Vivier wrote:
>> Build the Ethernet, IP, and TCP headers on the stack instead of
>> directly in the buffer via pointer casts, then write them into the
>> iovec with IOV_PUSH_HEADER().  This mirrors the approach already used
>> in tcp_vu_prepare() and udp_vu_prepare().
>>
>> Remove the vu_eth(), vu_ip(), vu_payloadv4() and vu_payloadv6() helpers
>> from vu_common.h, as they are no longer used anywhere.
>>
>> Introduce tcp_vu_send_dup() to handle DUP_ACK duplication using
>> vu_collect() and iov_memcpy() instead of a plain memcpy(), so that
>> the duplicated frame is also properly scattered across multiple iovecs.
>>
>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>> Reviewed-by: Jon Maloy <jmaloy@redhat.com>
> 
> One query below.  But at worst it can be addressed as a follow up, so
> 
> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> 
>> ---
>>   iov.c       |   1 -
>>   tcp_vu.c    | 156 ++++++++++++++++++++++++++++++----------------------
>>   vu_common.h |  20 -------
>>   3 files changed, 91 insertions(+), 86 deletions(-)
>>
>> diff --git a/iov.c b/iov.c
>> index 9248ba95a9f2..6fd684ad1a6b 100644
>> --- a/iov.c
>> +++ b/iov.c
>> @@ -208,7 +208,6 @@ void iov_memset(const struct iovec *iov, size_t iov_cnt, size_t offset, int c,
>>    *
>>    * Return: total number of bytes copied
>>    */
>> -/* cppcheck-suppress unusedFunction */
>>   size_t iov_memcpy(struct iovec *dst_iov, size_t dst_iov_cnt, size_t dst_offset,
>>   		  const struct iovec *src_iov, size_t src_iov_cnt,
>>   		  size_t src_offset, size_t length)
>> diff --git a/tcp_vu.c b/tcp_vu.c
>> index 55242ec40410..30633760e4ec 100644
>> --- a/tcp_vu.c
>> +++ b/tcp_vu.c
>> @@ -74,6 +74,43 @@ static size_t tcp_vu_hdrlen(bool v6)
>>   	return hdrlen;
>>   }
>>   
>> +/**
>> + * tcp_vu_send_dup() - Duplicate a frame into a new virtqueue element
>> + * @c:		Execution context
>> + * @vq:		Receive virtqueue
>> + * @dest_elem:	Destination virtqueue element to collect
>> + * @dest_iov:	Destination iovec array for collected buffers
>> + * @max_dest_iov: Maximum number of entries in @dest_iov
>> + * @src_iov:	Source iovec array containing the frame to duplicate
>> + * @src_cnt:	Number of entries in @src_iov
>> + * @vnlen:	Total frame length including virtio-net header
>> + *
>> + * Return: number of virtqueue elements collected (0 if none available)
>> + */
>> +static int tcp_vu_send_dup(const struct ctx *c, struct vu_virtq *vq,
>> +			   struct vu_virtq_element *dest_elem,
>> +			   struct iovec *dest_iov, size_t max_dest_iov,
>> +			   const struct iovec *src_iov, size_t src_cnt,
>> +			   size_t vnlen)
>> +{
>> +	const struct vu_dev *vdev = c->vdev;
>> +	size_t dest_cnt;
>> +	int elem_cnt;
>> +
>> +	elem_cnt = vu_collect(vdev, vq, dest_elem, 1, dest_iov, max_dest_iov,
>> +			      &dest_cnt, vnlen, NULL);
> 
> This (and the one in tcp_vu_send_flag()) still allows a maximum of 1
> element, although it now allows multiple iovs. That differs from the
> main data path, as of the previous patch..  Is that intentional, or
> just a gap?

It's intentionnal, 1 element will always have enough space to send flags.

Thanks,
Laurent

> 
>> +	if (elem_cnt == 0)
>> +		return 0;
>> +
>> +	iov_memcpy(dest_iov, dest_cnt, 0, src_iov, src_cnt, 0,
>> +		   MAX(VNET_HLEN + ETH_ZLEN, vnlen));
>> +
>> +	if (*c->pcap)
>> +		pcap_iov(dest_iov, dest_cnt, VNET_HLEN, vnlen - VNET_HLEN);
>> +
>> +	return elem_cnt;
>> +}
>> +
>>   /**
>>    * tcp_vu_send_flag() - Send segment with flags to vhost-user (no payload)
>>    * @c:		Execution context
>> @@ -88,97 +125,86 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
>>   {
>>   	struct vu_dev *vdev = c->vdev;
>>   	struct vu_virtq *vq = &vdev->vq[VHOST_USER_RX_QUEUE];
>> +	size_t optlen, hdrlen, iov_cnt, iov_used;
>>   	struct vu_virtq_element flags_elem[2];
>> -	size_t optlen, hdrlen, l2len;
>> -	struct ipv6hdr *ip6h = NULL;
>> -	struct iphdr *ip4h = NULL;
>> -	struct iovec flags_iov[2];
>> -	struct tcp_syn_opts *opts;
>> -	struct iov_tail payload;
>> -	struct tcphdr *th;
>> -	struct ethhdr *eh;
>> +	struct iov_tail payload, l2frame;
>> +	int elem_cnt, dup_elem_cnt = 0;
>> +	struct iovec flags_iov[64];
>> +	struct tcp_syn_opts opts;
>> +	struct tcphdr th = { 0 };
>> +	struct ipv6hdr ip6h;
>> +	struct iphdr ip4h;
>> +	struct ethhdr eh;
>>   	uint32_t seq;
>> -	int elem_cnt;
>>   	int ret;
>>   
>>   	hdrlen = tcp_vu_hdrlen(CONN_V6(conn));
>>   
>>   	elem_cnt = vu_collect(vdev, vq, &flags_elem[0], 1,
>> -			      &flags_iov[0], 1, NULL,
>> -			      hdrlen + sizeof(*opts), NULL);
>> -	if (elem_cnt != 1)
>> +			      flags_iov, ARRAY_SIZE(flags_iov), &iov_cnt,
>> +			      hdrlen + sizeof(opts), NULL);
>> +	if (elem_cnt == 0)
>>   		return -EAGAIN;
>>   
>> -	assert(flags_elem[0].in_num == 1);
>> -	assert(flags_elem[0].in_sg[0].iov_len >=
>> -	       MAX(hdrlen + sizeof(*opts), ETH_ZLEN + VNET_HLEN));
>> -
>> -	eh = vu_eth(flags_elem[0].in_sg[0].iov_base);
>> -
>> -	memcpy(eh->h_dest, c->guest_mac, sizeof(eh->h_dest));
>> -	memcpy(eh->h_source, c->our_tap_mac, sizeof(eh->h_source));
>> -
>> -	if (CONN_V4(conn)) {
>> -		eh->h_proto = htons(ETH_P_IP);
>> -
>> -		ip4h = vu_ip(flags_elem[0].in_sg[0].iov_base);
>> -		*ip4h = (struct iphdr)L2_BUF_IP4_INIT(IPPROTO_TCP);
>> -
>> -		th = vu_payloadv4(flags_elem[0].in_sg[0].iov_base);
>> -	} else {
>> -		eh->h_proto = htons(ETH_P_IPV6);
>> -
>> -		ip6h = vu_ip(flags_elem[0].in_sg[0].iov_base);
>> -		*ip6h = (struct ipv6hdr)L2_BUF_IP6_INIT(IPPROTO_TCP);
>> -		th = vu_payloadv6(flags_elem[0].in_sg[0].iov_base);
>> -	}
>> +	memcpy(eh.h_dest, c->guest_mac, sizeof(eh.h_dest));
>>   
>> -	memset(th, 0, sizeof(*th));
>> -	th->doff = sizeof(*th) / 4;
>> -	th->ack = 1;
>> +	if (CONN_V4(conn))
>> +		ip4h = (struct iphdr)L2_BUF_IP4_INIT(IPPROTO_TCP);
>> +	else
>> +		ip6h = (struct ipv6hdr)L2_BUF_IP6_INIT(IPPROTO_TCP);
>>   
>>   	seq = conn->seq_to_tap;
>> -	opts = (struct tcp_syn_opts *)(th + 1);
>> -	ret = tcp_prepare_flags(c, conn, flags, th, opts, &optlen);
>> +	ret = tcp_prepare_flags(c, conn, flags, &th, &opts, &optlen);
>>   	if (ret <= 0) {
>> -		vu_queue_rewind(vq, 1);
>> +		vu_queue_rewind(vq, elem_cnt);
>>   		return ret;
>>   	}
>>   
>> -	payload = IOV_TAIL(flags_elem[0].in_sg, 1, hdrlen);
>> -
>>   	if (flags & KEEPALIVE)
>>   		seq--;
>>   
>> -	tcp_fill_headers(c, conn, eh, ip4h, ip6h, th, &payload,
>> +	iov_used = iov_skip_bytes(flags_iov, iov_cnt,
>> +				  MAX(optlen + hdrlen, VNET_HLEN + ETH_ZLEN),
>> +				  NULL);
>> +	if (iov_used < iov_cnt)
>> +		iov_used++;
>> +	iov_cnt = iov_used;
>> +
>> +	payload = IOV_TAIL(flags_elem[0].in_sg, iov_cnt, hdrlen);
>> +	iov_from_buf(payload.iov, payload.cnt, payload.off, &opts, optlen);
>> +	tcp_fill_headers(c, conn, &eh, CONN_V4(conn) ? &ip4h : NULL,
>> +			 CONN_V6(conn) ? &ip6h : NULL, &th, &payload,
>>   			 optlen, IP4_CSUM | (*c->pcap ? TCP_CSUM : 0), seq);
>>   
>> -	vu_pad(flags_elem[0].in_sg, 1, hdrlen + optlen);
>> -	vu_flush(vdev, vq, flags_elem, 1, hdrlen + optlen);
>> +	vu_pad(flags_elem[0].in_sg, iov_cnt, hdrlen + optlen);
>> +
>> +	/* write headers */
>> +	l2frame = IOV_TAIL(flags_elem[0].in_sg, iov_cnt, VNET_HLEN);
>> +
>> +	IOV_PUSH_HEADER(&l2frame, eh);
>> +	if (CONN_V4(conn))
>> +		IOV_PUSH_HEADER(&l2frame, ip4h);
>> +	else
>> +		IOV_PUSH_HEADER(&l2frame, ip6h);
>> +	IOV_PUSH_HEADER(&l2frame, th);
>>   
>> -	l2len = hdrlen + optlen - VNET_HLEN;
>>   	if (*c->pcap)
>> -		pcap_iov(&flags_elem[0].in_sg[0], 1, VNET_HLEN, l2len);
>> +		pcap_iov(flags_elem[0].in_sg, iov_cnt, VNET_HLEN,
>> +			 hdrlen + optlen - VNET_HLEN);
>>   
>>   	if (flags & DUP_ACK) {
>> -		elem_cnt = vu_collect(vdev, vq, &flags_elem[1], 1,
>> -				      &flags_iov[1], 1, NULL,
>> -				      hdrlen + optlen, NULL);
>> -		if (elem_cnt == 1 &&
>> -		    flags_elem[1].in_sg[0].iov_len >=
>> -		    flags_elem[0].in_sg[0].iov_len) {
>> -			memcpy(flags_elem[1].in_sg[0].iov_base,
>> -			       flags_elem[0].in_sg[0].iov_base,
>> -			       flags_elem[0].in_sg[0].iov_len);
>> -
>> -			vu_flush(vdev, vq, &flags_elem[1], 1, hdrlen + optlen);
>> -
>> -			if (*c->pcap) {
>> -				pcap_iov(&flags_elem[1].in_sg[0], 1, VNET_HLEN,
>> -					 l2len);
>> -			}
>> -		}
>> +		dup_elem_cnt = tcp_vu_send_dup(c, vq, &flags_elem[elem_cnt],
>> +					       &flags_iov[iov_cnt],
>> +					       ARRAY_SIZE(flags_iov) - iov_cnt,
>> +					       flags_elem[0].in_sg, iov_cnt,
>> +					       hdrlen + optlen);
>>   	}
>> +	vu_flush(vdev, vq, flags_elem, elem_cnt, hdrlen + optlen);
>> +	if (dup_elem_cnt) {
>> +		vu_flush(vdev, vq, &flags_elem[elem_cnt], dup_elem_cnt,
>> +			 hdrlen + optlen);
>> +	}
>> +
>>   	vu_queue_notify(vdev, vq);
>>   
>>   	return 0;
>> diff --git a/vu_common.h b/vu_common.h
>> index 51f70084a7cb..817384175a1d 100644
>> --- a/vu_common.h
>> +++ b/vu_common.h
>> @@ -15,26 +15,6 @@
>>   #include "ip.h"
>>   #include "virtio.h"
>>   
>> -static inline void *vu_eth(void *base)
>> -{
>> -	return ((char *)base + VNET_HLEN);
>> -}
>> -
>> -static inline void *vu_ip(void *base)
>> -{
>> -	return (struct ethhdr *)vu_eth(base) + 1;
>> -}
>> -
>> -static inline void *vu_payloadv4(void *base)
>> -{
>> -	return (struct iphdr *)vu_ip(base) + 1;
>> -}
>> -
>> -static inline void *vu_payloadv6(void *base)
>> -{
>> -	return (struct ipv6hdr *)vu_ip(base) + 1;
>> -}
>> -
>>   int vu_collect(const struct vu_dev *vdev, struct vu_virtq *vq,
>>   	       struct vu_virtq_element *elem, int max_elem,
>>   	       struct iovec *in_sg, size_t max_in_sg, size_t *in_total,
>> -- 
>> 2.54.0
>>
>