[PATCH V2 net-next 2/2] vhost-net: reduce one userspace copy when building XDP buff
From: Jason Wang <jasowang@redhat.com>
Date: 2025-06-26 02:15:09
Also in:
kvm, lkml, virtualization
Subsystem:
the rest, virtio host (vhost) · Maintainers:
Linus Torvalds, "Michael S. Tsirkin", 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. Acked-by: Willem de Bruijn <willemb@google.com> Signed-off-by: Jason Wang <jasowang@redhat.com> --- Changes since V1: - Add a comment to explain no overlapping when using memcpy --- drivers/vhost/net.c | 14 +++++--------- 1 file changed, 5 insertions(+), 9 deletions(-)
diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 777eb6193985..a33a32a1e488 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 +715,8 @@ 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);
--
2.34.1