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

[Arseniy Krasnov <avkrasnov@rulkc.org>]

6/23/26 20:26, Michael S. Tsirkin wrote:
> 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?

Yes, this case is currently for io_uring only, because io_uring doesn't set SOCK_ZEROCOPY to perform zerocopy transmission.

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

Agree, I need to update commit message again :)

Thanks

>
>
>> +		} 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