Re: [RFC PATCH v4 03/17] vsock/virtio: support to send non-linear skb

[Stefano Garzarella]

On Sat, Jun 03, 2023 at 11:49:25PM +0300, Arseniy Krasnov wrote:
> For non-linear skb use its pages from fragment array as buffers in
> virtio tx queue. These pages are already pinned by 'get_user_pages()'
> during such skb creation.
>
> Signed-off-by: Arseniy Krasnov <AVKrasnov@xxxxxxxxxxxxxx>
> ---
> net/vmw_vsock/virtio_transport.c | 37 ++++++++++++++++++++++++++------
> 1 file changed, 31 insertions(+), 6 deletions(-)
>
> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> index e95df847176b..6053d8341091 100644
> --- a/net/vmw_vsock/virtio_transport.c
> +++ b/net/vmw_vsock/virtio_transport.c
> @@ -100,7 +100,9 @@ virtio_transport_send_pkt_work(struct work_struct *work)
> 	vq = vsock->vqs[VSOCK_VQ_TX];
>
> 	for (;;) {
> -		struct scatterlist hdr, buf, *sgs[2];
> +		/* +1 is for packet header. */
> +		struct scatterlist *sgs[MAX_SKB_FRAGS + 1];
> +		struct scatterlist bufs[MAX_SKB_FRAGS + 1];
> 		int ret, in_sg = 0, out_sg = 0;
> 		struct sk_buff *skb;
> 		bool reply;
> @@ -111,12 +113,35 @@ 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];

Can we use out_sg also to index bufs (here and in the rest of the code)?

E.g.

		sg_init_one(&bufs[out_sg], ...)
		sgs[out_sg] = &bufs[out_sg];
		++out_sg;

		...
			if (skb->len > 0) {
				sg_init_one(&bufs[out_sg], skb->data, skb->len);
				sgs[out_sg] = &bufs[out_sg];
				++out_sg;
			}

		etc...

> +

For readability, I would move the smaller branch above:

		if (!skb_is_nonlinear(skb)) {
			// small block
			...
		} else {
			// big block
			...
		}

> +		if (skb_is_nonlinear(skb)) {
> +			struct skb_shared_info *si;
> +			int i;
> +
> +			si = skb_shinfo(skb);
> +
> +			for (i = 0; i < si->nr_frags; i++) {
> +				skb_frag_t *skb_frag = &si->frags[i];
> +				void *va = page_to_virt(skb_frag->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.
> +				 */
> +				sg_init_one(&bufs[i + 1],
> +					    va + skb_frag->bv_offset,
> +					    skb_frag->bv_len);
> +				sgs[out_sg++] = &bufs[i + 1];
> +			}
> +		} else {
> +			if (skb->len > 0) {

Should we do the same check (skb->len > 0) for nonlinear skb as well?
Or do the nonlinear ones necessarily have len > 0?

> +				sg_init_one(&bufs[1], skb->data, skb->len);
> +				sgs[out_sg++] = &bufs[1];
> +			}
   ^
Blank line that we can remove.

Stefano

> -		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