Re: [PATCH net-next v3] vsock/virtio: rewrite MSG_ZEROCOPY flag handling

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 23, 2026 at 06:38:19PM +0300, Arseniy Krasnov wrote:
> Logically it was based on TCP implementation, so to make further support
> easier, rewrite it in the TCP way (like in 'tcp_sendmsg_locked()'). This
> patch only rewrites flag handling (e.g. it doesn't change logic).
> 
> Signed-off-by: Arseniy Krasnov <avkrasnov@rulkc.org>


It seems to change logic though:

> ---
>  Changelog v1->v2:
>  * Rebase on last 'net-next'. Don't need 'skb_zcopy_set()' now - it was
>    already added.
>  Changelog v2->v3:
>  * Update commit message.
>  * Remove one empty line.
> 
>  net/vmw_vsock/virtio_transport_common.c | 47 ++++++++++++-------------
>  1 file changed, 22 insertions(+), 25 deletions(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index 09475007165b..41c2a0b82a8e 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -328,38 +328,35 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>  	if (pkt_len == 0 && info->op == VIRTIO_VSOCK_OP_RW)
>  		return pkt_len;
>  
> -	if (info->msg) {
> -		/* If zerocopy is not enabled by 'setsockopt()', we behave as
> -		 * there is no MSG_ZEROCOPY flag set.
> +	if (info->msg && (info->msg->msg_flags & MSG_ZEROCOPY)) {
> +		/* If 'info->msg' is not NULL, this is only VIRTIO_VSOCK_OP_RW.
> +		 * 'MSG_ZEROCOPY' flag handling here is based on the same flag
> +		 * handling from 'tcp_sendmsg_locked()'.
>  		 */
> -		if (!sock_flag(sk_vsock(vsk), SOCK_ZEROCOPY))
> -			info->msg->msg_flags &= ~MSG_ZEROCOPY;

So previously without SOCK_ZEROCOPY, MSG_ZEROCOPY was always ignored...


> +		if (info->msg->msg_ubuf) {
> +			uarg = info->msg->msg_ubuf;
> +			can_zcopy = virtio_transport_can_zcopy(t_ops, info, pkt_len);

now it's not in this case?


Maybe the right call, but saying "does not change logic" seems wrong.


> +		} else if (sock_flag(sk_vsock(vsk), SOCK_ZEROCOPY)) {
> +			uarg = msg_zerocopy_realloc(sk_vsock(vsk), pkt_len,
> +						    NULL, false);
> +			if (!uarg) {
> +				virtio_transport_put_credit(vvs, pkt_len);
> +				return -ENOMEM;
> +			}
>  
> -		if (info->msg->msg_flags & MSG_ZEROCOPY)
>  			can_zcopy = virtio_transport_can_zcopy(t_ops, info, pkt_len);
> +			if (!can_zcopy)
> +				uarg_to_msgzc(uarg)->zerocopy = 0;
>  
> +			have_uref = true;
> +		}
> +
> +		/* 'can_zcopy' means that this transmission will be
> +		 * in zerocopy way (e.g. using 'frags' array).
> +		 */
>  		if (can_zcopy)
>  			max_skb_len = min_t(u32, VIRTIO_VSOCK_MAX_PKT_BUF_SIZE,
>  					    (MAX_SKB_FRAGS * PAGE_SIZE));
> -
> -		if (info->msg->msg_flags & MSG_ZEROCOPY &&
> -		    info->op == VIRTIO_VSOCK_OP_RW) {
> -			uarg = info->msg->msg_ubuf;
> -
> -			if (!uarg) {
> -				uarg = msg_zerocopy_realloc(sk_vsock(vsk),
> -							    pkt_len, NULL, false);
> -				if (!uarg) {
> -					virtio_transport_put_credit(vvs, pkt_len);
> -					return -ENOMEM;
> -				}
> -
> -				if (!can_zcopy)
> -					uarg_to_msgzc(uarg)->zerocopy = 0;
> -
> -				have_uref = true;
> -			}
> -		}
>  	}
>  
>  	rest_len = pkt_len;
> -- 
> 2.25.1