Re: [RFC PATCH 1/3] vu_common: Stick to size of input buffer in vu_send_single()

[Laurent Vivier <lvivier@redhat.com>]

On 11/3/25 11:16, Stefano Brivio wrote:
> ...instead of copying, from the input buffer, the amount of available
> bytes in the buffer provided via vhost-user.
> 
> The existing behaviour should be harmless due to the fact that we
> don't request overly large buffers from vhost-user, but it doesn't
> look correct to me.

I don't understand where is the problem.

I think the existing behaviour is correct because the iov array is padded to size in 
vu_collect() by:

                 if (iov->iov_len > size - current_size)
                         iov->iov_len = size - current_size;

total can be lesser than size but in this case we exit.

So total should be equal to input size + sizeof(struct virtio_net_hdr_mrg_rxbuf).

And in iov_from_buf() we copy the content of the buffer according the available room in 
the iovec and the size of the input buffer.

Thanks,
Laurent>
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> ---
>   vu_common.c | 22 +++++++++++-----------
>   1 file changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/vu_common.c b/vu_common.c
> index b13b7c3..21cfb2a 100644
> --- a/vu_common.c
> +++ b/vu_common.c
> @@ -238,7 +238,7 @@ void vu_kick_cb(struct vu_dev *vdev, union epoll_ref ref,
>    * vu_send_single() - Send a buffer to the front-end using the RX virtqueue
>    * @c:		execution context
>    * @buf:	address of the buffer
> - * @size:	size of the buffer
> + * @size:	size of the input buffer
>    *
>    * Return: number of bytes sent, -1 if there is an error
>    */
> @@ -248,7 +248,7 @@ int vu_send_single(const struct ctx *c, const void *buf, size_t size)
>   	struct vu_virtq *vq = &vdev->vq[VHOST_USER_RX_QUEUE];
>   	struct vu_virtq_element elem[VIRTQUEUE_MAX_SIZE];
>   	struct iovec in_sg[VIRTQUEUE_MAX_SIZE];
> -	size_t total;
> +	size_t vu_buf_size;
>   	int elem_cnt;
>   	int i;
>   
> @@ -261,21 +261,20 @@ int vu_send_single(const struct ctx *c, const void *buf, size_t size)
>   
>   	vu_init_elem(elem, in_sg, VIRTQUEUE_MAX_SIZE);
>   
> -	size += sizeof(struct virtio_net_hdr_mrg_rxbuf);
> -	elem_cnt = vu_collect(vdev, vq, elem, VIRTQUEUE_MAX_SIZE, size, &total);
> -	if (total < size) {
> +	elem_cnt = vu_collect(vdev, vq, elem, VIRTQUEUE_MAX_SIZE,
> +			      size + sizeof(struct virtio_net_hdr_mrg_rxbuf),
> +			      &vu_buf_size);
> +	if (vu_buf_size < size + sizeof(struct virtio_net_hdr_mrg_rxbuf)) {
>   		debug("vu_send_single: no space to send the data "
> -		      "elem_cnt %d size %zd", elem_cnt, total);
> +		      "elem_cnt %d size %zd", elem_cnt, size);
>   		goto err;
>   	}
>   
>   	vu_set_vnethdr(vdev, in_sg[0].iov_base, elem_cnt);
>   
> -	total -= sizeof(struct virtio_net_hdr_mrg_rxbuf);
> -
>   	/* copy data from the buffer to the iovec */
>   	iov_from_buf(in_sg, elem_cnt, sizeof(struct virtio_net_hdr_mrg_rxbuf),
> -		     buf, total);
> +		     buf, size);
>   
>   	if (*c->pcap) {
>   		pcap_iov(in_sg, elem_cnt,
> @@ -284,9 +283,10 @@ int vu_send_single(const struct ctx *c, const void *buf, size_t size)
>   
>   	vu_flush(vdev, vq, elem, elem_cnt);
>   
> -	trace("vhost-user sent %zu", total);
> +	trace("vhost-user sent %zu",
> +	      vu_buf_size - sizeof(struct virtio_net_hdr_mrg_rxbuf));
>   
> -	return total;
> +	return size;
>   err:
>   	for (i = 0; i < elem_cnt; i++)
>   		vu_queue_detach_element(vq);