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

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Date: Mon, 26 Jun 2023 17:36:52 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Arseniy Krasnov <AVKrasnov@...rdevices.ru>
Cc: Stefan Hajnoczi <stefanha@...hat.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>, 
	Bobby Eshleman <bobby.eshleman@...edance.com>, kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org, kernel@...rdevices.ru, 
	oxffffaa@...il.com
Subject: Re: [RFC PATCH v4 03/17] vsock/virtio: support to send non-linear skb

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@...rdevices.ru>
>---
> 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
>