Re: [RFC PATCH v3 05/17] vsock/virtio: MSG_ZEROCOPY flag support

[Simon Horman <simon.horman@corigine.com>]

On Mon, May 22, 2023 at 10:39:38AM +0300, Arseniy Krasnov wrote:
> This adds handling of MSG_ZEROCOPY flag on transmission path: if this
> flag is set and zerocopy transmission is possible, then non-linear skb
> will be created and filled with the pages of user's buffer. Pages of
> user's buffer are locked in memory by 'get_user_pages()'.
> 
> Signed-off-by: Arseniy Krasnov <AVKrasnov@sberdevices.ru>
> ---
>  net/vmw_vsock/virtio_transport_common.c | 305 +++++++++++++++++++-----
>  1 file changed, 243 insertions(+), 62 deletions(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index 9854f48a0544..5acf824afe41 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -37,73 +37,161 @@ virtio_transport_get_ops(struct vsock_sock *vsk)
>  	return container_of(t, struct virtio_transport, transport);
>  }
>  
> -/* Returns a new packet on success, otherwise returns NULL.
> - *
> - * If NULL is returned, errp is set to a negative errno.
> - */
> -static struct sk_buff *
> -virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *info,
> -			   size_t len,
> -			   u32 src_cid,
> -			   u32 src_port,
> -			   u32 dst_cid,
> -			   u32 dst_port)
> -{
> -	const size_t skb_len = VIRTIO_VSOCK_SKB_HEADROOM + len;
> -	struct virtio_vsock_hdr *hdr;
> -	struct sk_buff *skb;
> -	void *payload;
> -	int err;
> +static bool virtio_transport_can_zcopy(struct virtio_vsock_pkt_info *info,
> +				       size_t max_to_send)
> +{
> +	struct iov_iter *iov_iter;
> +	size_t max_skb_cap;
> +	size_t bytes;
> +	int i;
>  
> -	skb = virtio_vsock_alloc_skb(skb_len, GFP_KERNEL);
> -	if (!skb)
> -		return NULL;
> +	if (!info->msg)
> +		return false;
>  
> -	hdr = virtio_vsock_hdr(skb);
> -	hdr->type	= cpu_to_le16(info->type);
> -	hdr->op		= cpu_to_le16(info->op);
> -	hdr->src_cid	= cpu_to_le64(src_cid);
> -	hdr->dst_cid	= cpu_to_le64(dst_cid);
> -	hdr->src_port	= cpu_to_le32(src_port);
> -	hdr->dst_port	= cpu_to_le32(dst_port);
> -	hdr->flags	= cpu_to_le32(info->flags);
> -	hdr->len	= cpu_to_le32(len);
> +	if (!(info->flags & MSG_ZEROCOPY) && !info->msg->msg_ubuf)
> +		return false;
>  
> -	if (info->msg && len > 0) {
> -		payload = skb_put(skb, len);
> -		err = memcpy_from_msg(payload, info->msg, len);
> -		if (err)
> -			goto out;
> +	iov_iter = &info->msg->msg_iter;
> +
> +	if (iter_is_ubuf(iov_iter)) {
> +		if (offset_in_page(iov_iter->ubuf))
> +			return false;
> +
> +		return true;
> +	}
> +
> +	if (!iter_is_iovec(iov_iter))
> +		return false;
> +
> +	if (iov_iter->iov_offset)
> +		return false;
> +
> +	/* We can't send whole iov. */
> +	if (iov_iter->count > max_to_send)
> +		return false;
> +
> +	for (bytes = 0, i = 0; i < iov_iter->nr_segs; i++) {
> +		const struct iovec *iovec;
> +		int pages_in_elem;
> +
> +		iovec = &iov_iter->__iov[i];
> +
> +		/* Base must be page aligned. */
> +		if (offset_in_page(iovec->iov_base))
> +			return false;
>  
> -		if (msg_data_left(info->msg) == 0 &&
> -		    info->type == VIRTIO_VSOCK_TYPE_SEQPACKET) {
> -			hdr->flags |= cpu_to_le32(VIRTIO_VSOCK_SEQ_EOM);
> +		/* Only last element could have non page aligned size. */
> +		if (i != (iov_iter->nr_segs - 1)) {
> +			if (offset_in_page(iovec->iov_len))
> +				return false;
>  
> -			if (info->msg->msg_flags & MSG_EOR)
> -				hdr->flags |= cpu_to_le32(VIRTIO_VSOCK_SEQ_EOR);
> +			pages_in_elem = iovec->iov_len >> PAGE_SHIFT;
> +		} else {
> +			pages_in_elem = round_up(iovec->iov_len, PAGE_SIZE);
> +			pages_in_elem >>= PAGE_SHIFT;
>  		}
> +
> +		bytes += (pages_in_elem * PAGE_SIZE);
>  	}

Hi Arseniy,

bytes is set but the loop above, but seems otherwise unused in this function.

>  
> -	if (info->reply)
> -		virtio_vsock_skb_set_reply(skb);
> +	/* How many bytes we can pack to single skb. Maximum packet
> +	 * buffer size is needed to allow vhost handle such packets,
> +	 * otherwise they will be dropped.
> +	 */
> +	max_skb_cap = min((unsigned int)(MAX_SKB_FRAGS * PAGE_SIZE),
> +			  (unsigned int)VIRTIO_VSOCK_MAX_PKT_BUF_SIZE);

Likewise, max_skb_cap seems to be set but unused in this function.

>  
> -	trace_virtio_transport_alloc_pkt(src_cid, src_port,
> -					 dst_cid, dst_port,
> -					 len,
> -					 info->type,
> -					 info->op,
> -					 info->flags);
> +	return true;
> +}

...