[PATCH RFC v4 4/6] vhost-net: support VIRTIO_NET_F_RX_TSTAMP

[Steffen Trumtrar]

Advertise VIRTIO_NET_F_RX_TSTAMP to userspace and size the vnet header
for it, so a guest can negotiate the feature with a vhost-net backed
tap device.

The header is 40 bytes if the feature is negotiated. Whoever writes
the header (tun, or vhost itself with VHOST_NET_F_VIRTIO_NET_HDR) has
to fill in the timestamp; vhost only reserves the room. If vhost
writes the header, it zeroes everything behind num_buffers, so the
timestamp is 0, which the guest ignores.

Signed-off-by: Steffen Trumtrar <s.trumtrar@xxxxxxxxxxxxxx>
Assisted-by: Claude:claude-opus-5-5
---
 drivers/vhost/net.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 2cc730729e08a..2a38ad0ea1af2 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -77,7 +77,8 @@ static const int vhost_net_bits[] = {
 	VIRTIO_F_RING_RESET,
 	VIRTIO_F_IN_ORDER,
 	VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO,
-	VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO
+	VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO,
+	VIRTIO_NET_F_RX_TSTAMP
 };
 
 enum {
@@ -1282,6 +1283,16 @@ static void handle_rx(struct vhost_net *net)
 			vhost_discard_vq_desc(vq, headcount, ndesc);
 			goto out;
 		}
+		/* Zero the rest of a header supplied by vhost, e.g. tstamp */
+		if (unlikely(vhost_hlen > sizeof(hdr) + sizeof(num_buffers))) {
+			size_t rest = vhost_hlen - sizeof(hdr) - sizeof(num_buffers);
+
+			if (iov_iter_zero(rest, &fixup) != rest) {
+				vq_err(vq, "Failed vnet_hdr zeroing");
+				vhost_discard_vq_desc(vq, headcount, ndesc);
+				goto out;
+			}
+		}
 		nvq->done_idx += headcount;
 		count += in_order ? 1 : headcount;
 		if (nvq->done_idx > VHOST_NET_BATCH) {
@@ -1688,6 +1699,10 @@ static int vhost_net_set_features(struct vhost_net *n, const u64 *features)
 				     VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO))
 		hdr_len = sizeof(struct virtio_net_hdr_v1_hash_tunnel);
 
+	/* The header layout is cumulative, tstamp sits behind all other fields. */
+	if (virtio_features_test_bit(features, VIRTIO_NET_F_RX_TSTAMP))
+		hdr_len = offsetofend(struct virtio_net_common_hdr, tstamp);
+
 	if (virtio_features_test_bit(features, VHOST_NET_F_VIRTIO_NET_HDR)) {
 		/* vhost provides vnet_hdr */
 		vhost_hlen = hdr_len;

-- 
2.52.0