linux-kernel - Re: [RFC PATCH v4 02/17] vhost/vsock: read data from non-linear skb

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Date:   Mon, 26 Jun 2023 17:24:07 +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 02/17] vhost/vsock: read data from non-linear skb

On Sat, Jun 03, 2023 at 11:49:24PM +0300, 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. It
>changes call of 'copy_to_iter()' to 'skb_copy_datagram_iter()'. Second
>function can read data from non-linear skb.
>
>See commit to 'net/vmw_vsock/virtio_transport_common.c' with the same
>name for more details.

I think it's okay if we report the same details here.

>
>Signed-off-by: Arseniy Krasnov <AVKrasnov@...rdevices.ru>
>---
> drivers/vhost/vsock.c | 12 +++++++-----
> 1 file changed, 7 insertions(+), 5 deletions(-)
>
>diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
>index 6578db78f0ae..b254aa4b756a 100644
>--- a/drivers/vhost/vsock.c
>+++ b/drivers/vhost/vsock.c
>@@ -156,7 +156,7 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
> 		}
>
> 		iov_iter_init(&iov_iter, ITER_DEST, &vq->iov[out], in, iov_len);
>-		payload_len = skb->len;
>+		payload_len = skb->len - VIRTIO_VSOCK_SKB_CB(skb)->frag_off;

Also here a variable should make the code more readable.

Stefano

> 		hdr = virtio_vsock_hdr(skb);
>
> 		/* If the packet is greater than the space available in the
>@@ -197,8 +197,10 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
> 			break;
> 		}
>
>-		nbytes = copy_to_iter(skb->data, payload_len, &iov_iter);
>-		if (nbytes != payload_len) {
>+		if (skb_copy_datagram_iter(skb,
>+					   VIRTIO_VSOCK_SKB_CB(skb)->frag_off,
>+					   &iov_iter,
>+					   payload_len)) {
> 			kfree_skb(skb);
> 			vq_err(vq, "Faulted on copying pkt buf\n");
> 			break;
>@@ -212,13 +214,13 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
> 		vhost_add_used(vq, head, sizeof(*hdr) + payload_len);
> 		added = true;
>
>-		skb_pull(skb, payload_len);
>+		VIRTIO_VSOCK_SKB_CB(skb)->frag_off += payload_len;
> 		total_len += payload_len;
>
> 		/* If we didn't send all the payload we can requeue the packet
> 		 * to send it with the next available buffer.
> 		 */
>-		if (skb->len > 0) {
>+		if (VIRTIO_VSOCK_SKB_CB(skb)->frag_off < skb->len) {
> 			hdr->flags |= cpu_to_le32(flags_to_restore);
>
> 			/* We are queueing the same skb to handle
>-- 
>2.25.1
>