Jason Wang wrote:
> 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>
Acked-by: Willem de Bruijn <will...@google.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);
It's not trivial to see that the dst and src do not overlap, and does
does not need memmove.
Minimal pad that I can find is 32B and and maximal sock_hlen is 12B.
So this is safe. But not obviously so. Unfortunately, these offsets
are not all known at compile time, so a BUILD_BUG_ON is not possible.
> xdp_init_buff(xdp, buflen, NULL);
> xdp_prepare_buff(xdp, buf, pad, len, true);
> --
> 2.34.1
>
