Re: [PATCH v2 13/13] vhost-user,udp: Use 2 iovec entries per element

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Mar 09, 2026 at 10:47:44AM +0100, Laurent Vivier wrote:
> iPXE places the vnet header in one virtqueue descriptor and the payload
> in another. When passt maps these descriptors, it needs two iovecs per
> virtqueue element to handle this layout.
> 
> Without this, passt crashes with:
> 
>   ASSERTION FAILED in virtqueue_map_desc (virtio.c:403): num_sg < max_num_sg
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  udp_vu.c    |  8 ++++----
>  vu_common.c | 15 +++++++++++----
>  2 files changed, 15 insertions(+), 8 deletions(-)
> 
> diff --git a/udp_vu.c b/udp_vu.c
> index cb5274aa1d81..47659b0402fd 100644
> --- a/udp_vu.c
> +++ b/udp_vu.c
> @@ -34,7 +34,7 @@
>  #include "vu_common.h"
>  
>  static struct iovec     iov_vu		[VIRTQUEUE_MAX_SIZE];
> -static struct vu_virtq_element	elem		[VIRTQUEUE_MAX_SIZE];
> +static struct vu_virtq_element	elem	[VIRTQUEUE_MAX_SIZE / IOV_PER_ELEM];

The two level structure of the queues (array of elems, each pointing
to an array of iovs) confuses me a bit.  I would have thought that
VIRTQUEUE_MAX_SIZE represented the maximum number of elements, even if
those elements had multiple iovs.  Can you enlighten me?

>  /**
>   * udp_vu_hdrlen() - Sum size of all headers, from UDP to virtio-net
> @@ -214,20 +214,20 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
>  		size_t iov_cnt;
>  		ssize_t dlen;
>  
> -		vu_init_elem(elem, iov_vu, ARRAY_SIZE(elem), 1);
> +		vu_init_elem(elem, iov_vu, ARRAY_SIZE(elem), IOV_PER_ELEM);
>  
>  		elem_cnt = vu_collect(vdev, vq, elem, ARRAY_SIZE(elem),
>  				      IP_MAX_MTU + ETH_HLEN + VNET_HLEN, NULL);
>  		if (elem_cnt == 0)
>  			break;
>  
> -		iov_cnt = elem_cnt;
> +		iov_cnt = (size_t)elem_cnt * IOV_PER_ELEM;
>  		dlen = udp_vu_sock_recv(iov_vu, &iov_cnt, s, v6);
>  		if (dlen < 0) {
>  			vu_queue_rewind(vq, elem_cnt);
>  			break;
>  		}
> -		elem_used = iov_cnt;
> +		elem_used = DIV_ROUND_UP(iov_cnt, IOV_PER_ELEM);
>  
>  		/* release unused buffers */
>  		vu_queue_rewind(vq, elem_cnt - elem_used);
> diff --git a/vu_common.c b/vu_common.c
> index 3225aca53ea6..a2867a293184 100644
> --- a/vu_common.c
> +++ b/vu_common.c
> @@ -63,8 +63,15 @@ void vu_init_elem(struct vu_virtq_element *elem, struct iovec *iov,
>  {
>  	int i, j;
>  
> -	for (i = 0, j = 0; i < elem_cnt; i++, j += iov_per_elem)
> +	for (i = 0, j = 0; i < elem_cnt; i++, j += iov_per_elem) {
> +		int k;
> +
> +		for (k = 0; k < iov_per_elem; k++) {
> +			iov[j + k].iov_base = NULL;
> +			iov[j + k].iov_len = 0;
> +		}
>  		vu_set_element(&elem[i], 0, NULL, iov_per_elem, &iov[j]);
> +	}
>  }
>  
>  /**
> @@ -272,7 +279,7 @@ int vu_send_single(const struct ctx *c, const void *buf, size_t size)
>  {
>  	struct vu_dev *vdev = c->vdev;
>  	struct vu_virtq *vq = &vdev->vq[VHOST_USER_RX_QUEUE];
> -	struct vu_virtq_element elem[VIRTQUEUE_MAX_SIZE];
> +	struct vu_virtq_element elem[VIRTQUEUE_MAX_SIZE / IOV_PER_ELEM];
>  	struct iovec in_sg[VIRTQUEUE_MAX_SIZE];
>  	struct iov_tail data;
>  	size_t total;
> @@ -286,7 +293,7 @@ int vu_send_single(const struct ctx *c, const void *buf, size_t size)
>  		return -1;
>  	}
>  
> -	vu_init_elem(elem, in_sg, ARRAY_SIZE(elem), 1);
> +	vu_init_elem(elem, in_sg, ARRAY_SIZE(elem), IOV_PER_ELEM);
>  
>  	size += VNET_HLEN;
>  	elem_cnt = vu_collect(vdev, vq, elem, ARRAY_SIZE(elem), size, &total);
> @@ -303,7 +310,7 @@ int vu_send_single(const struct ctx *c, const void *buf, size_t size)
>  	}
>  	elem_cnt = iov_truncate(in_sg, elem_cnt, size);
>  
> -	data = IOV_TAIL(&in_sg[0], elem_cnt, 0);
> +	data = IOV_TAIL(&in_sg[0], (size_t)(elem_cnt * IOV_PER_ELEM), 0);
>  	vu_set_vnethdr(&data, elem_cnt);
>  
>  	size -= VNET_HLEN;
> -- 
> 2.53.0
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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