netdev - Re: [RFC PATCH v4 04/17] vsock/virtio: non-linear skb handling for tap

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Date: Mon, 26 Jun 2023 17:43:14 +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 04/17] vsock/virtio: non-linear skb handling for
 tap

On Sat, Jun 03, 2023 at 11:49:26PM +0300, Arseniy Krasnov wrote:
>For tap device new skb is created and data from the current skb is
>copied to it. This adds copying data from non-linear skb to new
>the skb.
>
>Signed-off-by: Arseniy Krasnov <AVKrasnov@...rdevices.ru>
>---
> net/vmw_vsock/virtio_transport_common.c | 31 ++++++++++++++++++++++---
> 1 file changed, 28 insertions(+), 3 deletions(-)
>
>diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>index 5819a9cd4515..0de562c1dc4b 100644
>--- a/net/vmw_vsock/virtio_transport_common.c
>+++ b/net/vmw_vsock/virtio_transport_common.c
>@@ -106,6 +106,27 @@ virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *info,
> 	return NULL;
> }
>
>+static void virtio_transport_copy_nonlinear_skb(struct sk_buff *skb,

`const struct sk_buff *skb` should be better also to understand that
the function copy data from *skb to *dst.

>+						void *dst,
>+						size_t len)
>+{
>+	struct iov_iter iov_iter = { 0 };
>+	struct kvec kvec;
>+	size_t to_copy;
>+
>+	kvec.iov_base = dst;
>+	kvec.iov_len = len;
>+
>+	iov_iter.iter_type = ITER_KVEC;
>+	iov_iter.kvec = &kvec;
>+	iov_iter.nr_segs = 1;
>+
>+	to_copy = min_t(size_t, len, skb->len);
>+
>+	skb_copy_datagram_iter(skb, VIRTIO_VSOCK_SKB_CB(skb)->frag_off,
>+			       &iov_iter, to_copy);
>+}
>+
> /* Packet capture */
> static struct sk_buff *virtio_transport_build_skb(void *opaque)
> {
>@@ -114,7 +135,6 @@ static struct sk_buff *virtio_transport_build_skb(void *opaque)
> 	struct af_vsockmon_hdr *hdr;
> 	struct sk_buff *skb;
> 	size_t payload_len;
>-	void *payload_buf;
>
> 	/* A packet could be split to fit the RX buffer, so we can retrieve
> 	 * the payload length from the header and the buffer pointer taking
>@@ -122,7 +142,6 @@ static struct sk_buff *virtio_transport_build_skb(void *opaque)
> 	 */
> 	pkt_hdr = virtio_vsock_hdr(pkt);
> 	payload_len = pkt->len;
>-	payload_buf = pkt->data;
>
> 	skb = alloc_skb(sizeof(*hdr) + sizeof(*pkt_hdr) + payload_len,
> 			GFP_ATOMIC);
>@@ -165,7 +184,13 @@ static struct sk_buff *virtio_transport_build_skb(void *opaque)
> 	skb_put_data(skb, pkt_hdr, sizeof(*pkt_hdr));
>
> 	if (payload_len) {
>-		skb_put_data(skb, payload_buf, payload_len);
>+		if (skb_is_nonlinear(pkt)) {
>+			void *data = skb_put(skb, payload_len);
>+
>+			virtio_transport_copy_nonlinear_skb(pkt, data, 
>payload_len);
>+		} else {
>+			skb_put_data(skb, pkt->data, payload_len);
>+		}
> 	}
>
> 	return skb;
>-- 
>2.25.1
>