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

[Jason Wang]

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 <jasow...@redhat.com>
---
 drivers/vhost/net.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 777eb6193985..2845e0a473ea 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);
@@ -715,12 +715,7 @@ 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;
-       }
+       memcpy(buf, buf + pad - sock_hlen, sock_hlen);
 
        xdp_init_buff(xdp, buflen, NULL);
        xdp_prepare_buff(xdp, buf, pad, len, true);
-- 
2.34.1