[PATCH net-next V3 2/2] vhost-net: reduce one userspace copy when building XDP buff

[Jason Wang <jasowang@redhat.com>]

We used to do twice copy_from_iter() to copy virtio-net and packet
separately. This introduce overheads for userspace access hardening as
well as SMAP (for x86 it's stac/clac). So this patch tries to use one
copy_from_iter() to copy them once and move the virtio-net header
afterwards to reduce overheads.

Testpmd + vhost_net shows 10% improvement from 5.45Mpps to 6.0Mpps.

Signed-off-by: Jason Wang <jasowang@redhat.com>
---
Changes since V2:
- fix the data_len calculation
Changes since V1:
- add a comment to explain there's no overlapping
---
 drivers/vhost/net.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 777eb6193985..9dbd88eb9ff4 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -690,13 +690,13 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
 	if (unlikely(!buf))
 		return -ENOMEM;
 
-	copied = copy_from_iter(buf, sock_hlen, from);
-	if (copied != sock_hlen) {
+	copied = copy_from_iter(buf + pad - sock_hlen, len, from);
+	if (copied != len) {
 		ret = -EFAULT;
 		goto err;
 	}
 
-	gso = buf;
+	gso = buf + pad - sock_hlen;
 
 	if (!sock_hlen)
 		memset(buf, 0, pad);
@@ -715,15 +715,11 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
 		}
 	}
 
-	len -= sock_hlen;
-	copied = copy_from_iter(buf + pad, len, from);
-	if (copied != len) {
-		ret = -EFAULT;
-		goto err;
-	}
+	/* pad contains sock_hlen */
+	memcpy(buf, buf + pad - sock_hlen, sock_hlen);
 
 	xdp_init_buff(xdp, buflen, NULL);
-	xdp_prepare_buff(xdp, buf, pad, len, true);
+	xdp_prepare_buff(xdp, buf, pad, len - sock_hlen, true);
 
 	++nvq->batched_xdp;
 
-- 
2.34.1