Re: [RFC PATCH v5 03/17] vsock/virtio: support to send non-linear skb

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On Sat, Jul 01, 2023 at 09:39:33AM +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>
---
Changelog:
v4 -> v5:
 * Use 'out_sgs' variable to index 'bufs', not only 'sgs'.
 * Move smaller branch above, see 'if (!skb_is_nonlinear(skb)').
 * Remove blank line.
 * R-b from Bobby Eshleman removed due to patch update.

net/vmw_vsock/virtio_transport.c | 40 +++++++++++++++++++++++++++-----
1 file changed, 34 insertions(+), 6 deletions(-)

Reviewed-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>


diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index e95df847176b..6cbb45bb12d2 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 +113,38 @@ 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[out_sg], virtio_vsock_hdr(skb),
+			    sizeof(*virtio_vsock_hdr(skb)));
+		sgs[out_sg] = &bufs[out_sg];
+		out_sg++;
+
+		if (!skb_is_nonlinear(skb)) {
+			if (skb->len > 0) {
+				sg_init_one(&bufs[out_sg], skb->data, skb->len);
+				sgs[out_sg] = &bufs[out_sg];
+				out_sg++;
+			}
+		} else {
+			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);

-		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;
+				/* 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[out_sg],
+					    va + skb_frag->bv_offset,
+					    skb_frag->bv_len);
+				sgs[out_sg] = &bufs[out_sg];
+				out_sg++;
+			}
		}

		ret = virtqueue_add_sgs(vq, sgs, out_sg, in_sg, skb, GFP_KERNEL);
--
2.25.1





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