Re: [RFC PATCH v1 05/12] vsock/virtio: non-linear skb support

[Stefano Garzarella]

On Mon, Feb 06, 2023 at 06:58:24AM +0000, Arseniy Krasnov wrote:
> Use pages of non-linear skb as buffers in virtio tx queue.
>
> Signed-off-by: Arseniy Krasnov <AVKrasnov@xxxxxxxxxxxxxx>
> ---
> net/vmw_vsock/virtio_transport.c | 31 +++++++++++++++++++++++++------
> 1 file changed, 25 insertions(+), 6 deletions(-)
>
> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> index 28b5a8e8e094..b8a7d6dc9f46 100644
> --- a/net/vmw_vsock/virtio_transport.c
> +++ b/net/vmw_vsock/virtio_transport.c
> @@ -100,7 +100,8 @@ virtio_transport_send_pkt_work(struct work_struct *work)
> 	vq = vsock->vqs[VSOCK_VQ_TX];
>
> 	for (;;) {
> -		struct scatterlist hdr, buf, *sgs[2];
> +		struct scatterlist *sgs[MAX_SKB_FRAGS + 1];
> +		struct scatterlist bufs[MAX_SKB_FRAGS + 1];

+ 1 is for the header, right?
I'd add a comment just to be clear ;-)

> 		int ret, in_sg = 0, out_sg = 0;
> 		struct sk_buff *skb;
> 		bool reply;
> @@ -111,12 +112,30 @@ virtio_transport_send_pkt_work(struct work_struct *work)
>
> 		virtio_transport_deliver_tap_pkt(skb);
> 		reply = virtio_vsock_skb_reply(skb);
> +		sg_init_one(&bufs[0], virtio_vsock_hdr(skb), sizeof(*virtio_vsock_hdr(skb)));
> +		sgs[out_sg++] = &bufs[0];
> +
> +		if (skb_is_nonlinear(skb)) {
> +			int i;
> +
> +			for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
> +				struct page *data_page = skb_shinfo(skb)->frags[i].bv_page;
> +
> +				/* We will use 'page_to_virt()' for userspace page here,
> +				 * because virtio layer will call 'virt_to_phys()' later
> +				 * to fill buffer descriptor. We don't touch memory at
> +				 * "virtual" address of this page.
> +				 */

IIUC data_page is a user page, so since we are exposing it to the host,
I think we should pin it.

Is data_page always a user page, or can it be a kernel page when skb is 
nonlinear?

Thanks,
Stefano

> +				sg_init_one(&bufs[i + 1],
> +					    page_to_virt(data_page), PAGE_SIZE);
> +				sgs[out_sg++] = &bufs[i + 1];
> +			}
> +		} else {
> +			if (skb->len > 0) {
> +				sg_init_one(&bufs[1], skb->data, skb->len);
> +				sgs[out_sg++] = &bufs[1];
> +			}
>
> -		sg_init_one(&hdr, virtio_vsock_hdr(skb), sizeof(*virtio_vsock_hdr(skb)));
> -		sgs[out_sg++] = &hdr;
> -		if (skb->len > 0) {
> -			sg_init_one(&buf, skb->data, skb->len);
> -			sgs[out_sg++] = &buf;
> 		}
>
> 		ret = virtqueue_add_sgs(vq, sgs, out_sg, in_sg, skb, GFP_KERNEL);
> --
> 2.25.1