Re: [RFC PATCH v1 04/12] vhost/vsock: non-linear skb handling support

[Stefano Garzarella <sgarzare@xxxxxxxxxx>]

On Mon, Feb 06, 2023 at 06:57:16AM +0000, Arseniy Krasnov wrote:
> This adds copying to guest's virtio buffers from non-linear skbs. Such
> skbs are created by protocol layer when MSG_ZEROCOPY flags is used.
>
> Signed-off-by: Arseniy Krasnov <AVKrasnov@xxxxxxxxxxxxxx>
> ---
> drivers/vhost/vsock.c        | 56 ++++++++++++++++++++++++++++++++----
> include/linux/virtio_vsock.h | 12 ++++++++
> 2 files changed, 63 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> index 1f3b89c885cc..60b9cafa3e31 100644
> --- a/drivers/vhost/vsock.c
> +++ b/drivers/vhost/vsock.c
> @@ -86,6 +86,44 @@ static struct vhost_vsock *vhost_vsock_get(u32 guest_cid)
> 	return NULL;
> }
>
> +static int vhost_transport_copy_nonlinear_skb(struct sk_buff *skb,
> +					      struct iov_iter *iov_iter,
> +					      size_t len)
> +{
> +	size_t rest_len = len;
> +
> +	while (rest_len && virtio_vsock_skb_has_frags(skb)) {
> +		struct bio_vec *curr_vec;
> +		size_t curr_vec_end;
> +		size_t to_copy;
> +		int curr_frag;
> +		int curr_offs;
> +
> +		curr_frag = VIRTIO_VSOCK_SKB_CB(skb)->curr_frag;
> +		curr_offs = VIRTIO_VSOCK_SKB_CB(skb)->frag_off;
> +		curr_vec = &skb_shinfo(skb)->frags[curr_frag];
> +
> +		curr_vec_end = curr_vec->bv_offset + curr_vec->bv_len;
> +		to_copy = min(rest_len, (size_t)(curr_vec_end - curr_offs));
> +
> +		if (copy_page_to_iter(curr_vec->bv_page, curr_offs,
> +				      to_copy, iov_iter) != to_copy)
> +			return -1;
> +
> +		rest_len -= to_copy;
> +		VIRTIO_VSOCK_SKB_CB(skb)->frag_off += to_copy;
> +
> +		if (VIRTIO_VSOCK_SKB_CB(skb)->frag_off == (curr_vec_end)) {
> +			VIRTIO_VSOCK_SKB_CB(skb)->curr_frag++;
> +			VIRTIO_VSOCK_SKB_CB(skb)->frag_off = 0;
> +		}
> +	}

Can it happen that we exit this loop and rest_len is not 0?

In this case, is it correct to decrement data_len by len?

Thanks,
Stefano

> +
> +	skb->data_len -= len;
> +
> +	return 0;
> +}
> +
> static void
> vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
> 			    struct vhost_virtqueue *vq)
> @@ -197,11 +235,19 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
> 			break;
> 		}
>
> -		nbytes = copy_to_iter(skb->data, payload_len, &iov_iter);
> -		if (nbytes != payload_len) {
> -			kfree_skb(skb);
> -			vq_err(vq, "Faulted on copying pkt buf\n");
> -			break;
> +		if (skb_is_nonlinear(skb)) {
> +			if (vhost_transport_copy_nonlinear_skb(skb, &iov_iter,
> +							       payload_len)) {
> +				vq_err(vq, "Faulted on copying pkt buf from page\n");
> +				break;
> +			}
> +		} else {
> +			nbytes = copy_to_iter(skb->data, payload_len, &iov_iter);
> +			if (nbytes != payload_len) {
> +				kfree_skb(skb);
> +				vq_err(vq, "Faulted on copying pkt buf\n");
> +				break;
> +			}
> 		}
>
> 		/* Deliver to monitoring devices all packets that we
> diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
> index 3f9c16611306..e7efdb78ce6e 100644
> --- a/include/linux/virtio_vsock.h
> +++ b/include/linux/virtio_vsock.h
> @@ -12,6 +12,10 @@
> struct virtio_vsock_skb_cb {
> 	bool reply;
> 	bool tap_delivered;
> +	/* Current fragment in 'frags' of skb. */
> +	u32 curr_frag;
> +	/* Offset from 0 in current fragment. */
> +	u32 frag_off;
> };
>
> #define VIRTIO_VSOCK_SKB_CB(skb) ((struct virtio_vsock_skb_cb *)((skb)->cb))
> @@ -46,6 +50,14 @@ static inline void virtio_vsock_skb_clear_tap_delivered(struct sk_buff *skb)
> 	VIRTIO_VSOCK_SKB_CB(skb)->tap_delivered = false;
> }
>
> +static inline bool virtio_vsock_skb_has_frags(struct sk_buff *skb)
> +{
> +	if (!skb_is_nonlinear(skb))
> +		return false;
> +
> +	return VIRTIO_VSOCK_SKB_CB(skb)->curr_frag != skb_shinfo(skb)->nr_frags;
> +}
> +
> static inline void virtio_vsock_skb_rx_put(struct sk_buff *skb)
> {
> 	u32 len;
> --
> 2.25.1