Re: [RFC PATCH v1 06/12] vsock/virtio: non-linear skb handling for TAP dev

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On Mon, Feb 06, 2023 at 06:59:21AM +0000, 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@xxxxxxxxxxxxxx>
---
net/vmw_vsock/virtio_transport_common.c | 43 +++++++++++++++++++++++--
1 file changed, 40 insertions(+), 3 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index a1581c77cf84..05ce97b967ad 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -101,6 +101,39 @@ virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *info,
	return NULL;
}

+static void virtio_transport_copy_nonlinear_skb(struct sk_buff *skb,
+						void *dst,
+						size_t len)
+{
+	size_t rest_len = len;
+
+	while (rest_len && virtio_vsock_skb_has_frags(skb)) {
+		struct bio_vec *curr_vec;
+		size_t curr_vec_end;
+		size_t to_copy;
+		int curr_frag;
+		int curr_offs;
+
+		curr_frag = VIRTIO_VSOCK_SKB_CB(skb)->curr_frag;
+		curr_offs = VIRTIO_VSOCK_SKB_CB(skb)->frag_off;
+		curr_vec = &skb_shinfo(skb)->frags[curr_frag];
+
+		curr_vec_end = curr_vec->bv_offset + curr_vec->bv_len;
+		to_copy = min(rest_len, (size_t)(curr_vec_end - curr_offs));
+
+		memcpy(dst, page_to_virt(curr_vec->bv_page) + curr_offs,
+		       to_copy);
+
+		rest_len -= to_copy;
+		VIRTIO_VSOCK_SKB_CB(skb)->frag_off += to_copy;
+
+		if (VIRTIO_VSOCK_SKB_CB(skb)->frag_off == (curr_vec_end)) {
+			VIRTIO_VSOCK_SKB_CB(skb)->curr_frag++;
+			VIRTIO_VSOCK_SKB_CB(skb)->frag_off = 0;
+		}
+	}
+}
+
/* Packet capture */
static struct sk_buff *virtio_transport_build_skb(void *opaque)
{
@@ -109,7 +142,6 @@ 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
@@ -117,7 +149,6 @@ 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);
@@ -160,7 +191,13 @@ 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(skb)) {
+			void *data = skb_put(skb, payload_len);
+
+			virtio_transport_copy_nonlinear_skb(skb, data, payload_len);
+		} else {
+			skb_put_data(skb, pkt->data, payload_len);
+		}

Ehm I'm a bit confused. Maybe we need to rename the sk_buffs involved in
this function (pre-existing).

We have `pkt` that is the original sk_buff, and `skb` that it is allocated in this function, so IIUC we should check if `pkt` is nonlinear and copy its payload into `skb`, so we should do this (untested) chage:

@@ -367,10 +367,10 @@ static struct sk_buff *virtio_transport_build_skb(void *opaque)
        skb_put_data(skb, pkt_hdr, sizeof(*pkt_hdr));

        if (payload_len) {
-               if (skb_is_nonlinear(skb)) {
+               if (skb_is_nonlinear(pkt)) {
                        void *data = skb_put(skb, payload_len);

-                       virtio_transport_copy_nonlinear_skb(skb, data, payload_len);
+                       virtio_transport_copy_nonlinear_skb(pkt, data, payload_len);
                } else {
                        skb_put_data(skb, pkt->data, payload_len);
                }

Thanks,
Stefano

	}

	return skb;
--
2.25.1




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