[PATCH net-next 1/2] tun: remove unnecessary tun_xdp_hdr structure

[PATCH net-next 1/2] tun: remove unnecessary tun_xdp_hdr structure

[Jason Wang]

With f95f0f95cfb7("net, xdp: Introduce xdp_init_buff utility routine"),
buffer length could be stored as frame size so there's no need to have
a dedicated tun_xdp_hdr structure. We can simply store virtio net
header instead.

Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 drivers/net/tap.c      | 5 ++---
 drivers/net/tun.c      | 5 ++---
 drivers/vhost/net.c    | 8 ++------
 include/linux/if_tun.h | 5 -----
 4 files changed, 6 insertions(+), 17 deletions(-)

diff --git a/drivers/net/tap.c b/drivers/net/tap.c
index bdf0788d8e66..d82eb7276a8b 100644
--- a/drivers/net/tap.c
+++ b/drivers/net/tap.c
@@ -1044,9 +1044,8 @@ static const struct file_operations tap_fops = {
 
 static int tap_get_user_xdp(struct tap_queue *q, struct xdp_buff *xdp)
 {
-	struct tun_xdp_hdr *hdr = xdp->data_hard_start;
-	struct virtio_net_hdr *gso = &hdr->gso;
-	int buflen = hdr->buflen;
+	struct virtio_net_hdr *gso = xdp->data_hard_start;
+	int buflen = xdp->frame_sz;
 	int vnet_hdr_len = 0;
 	struct tap_dev *tap;
 	struct sk_buff *skb;
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 1207196cbbed..90055947a54b 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -2356,13 +2356,12 @@ static int tun_xdp_one(struct tun_struct *tun,
 		       struct tun_page *tpage)
 {
 	unsigned int datasize = xdp->data_end - xdp->data;
-	struct tun_xdp_hdr *hdr = xdp->data_hard_start;
-	struct virtio_net_hdr *gso = &hdr->gso;
+	struct virtio_net_hdr *gso = xdp->data_hard_start;
 	struct bpf_prog *xdp_prog;
 	struct sk_buff *skb = NULL;
 	struct sk_buff_head *queue;
 	u32 rxhash = 0, act;
-	int buflen = hdr->buflen;
+	int buflen = xdp->frame_sz;
 	int metasize = 0;
 	int ret = 0;
 	bool skb_xdp = false;
diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 7cbfc7d718b3..777eb6193985 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -668,7 +668,6 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
 	struct socket *sock = vhost_vq_get_backend(vq);
 	struct virtio_net_hdr *gso;
 	struct xdp_buff *xdp = &nvq->xdp[nvq->batched_xdp];
-	struct tun_xdp_hdr *hdr;
 	size_t len = iov_iter_count(from);
 	int headroom = vhost_sock_xdp(sock) ? XDP_PACKET_HEADROOM : 0;
 	int buflen = SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
@@ -691,15 +690,13 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
 	if (unlikely(!buf))
 		return -ENOMEM;
 
-	copied = copy_from_iter(buf + offsetof(struct tun_xdp_hdr, gso),
-				sock_hlen, from);
+	copied = copy_from_iter(buf, sock_hlen, from);
 	if (copied != sock_hlen) {
 		ret = -EFAULT;
 		goto err;
 	}
 
-	hdr = buf;
-	gso = &hdr->gso;
+	gso = buf;
 
 	if (!sock_hlen)
 		memset(buf, 0, pad);
@@ -727,7 +724,6 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
 
 	xdp_init_buff(xdp, buflen, NULL);
 	xdp_prepare_buff(xdp, buf, pad, len, true);
-	hdr->buflen = buflen;
 
 	++nvq->batched_xdp;
 
diff --git a/include/linux/if_tun.h b/include/linux/if_tun.h
index 043d442994b0..80166eb62f41 100644
--- a/include/linux/if_tun.h
+++ b/include/linux/if_tun.h
@@ -19,11 +19,6 @@ struct tun_msg_ctl {
 	void *ptr;
 };
 
-struct tun_xdp_hdr {
-	int buflen;
-	struct virtio_net_hdr gso;
-};
-
 #if defined(CONFIG_TUN) || defined(CONFIG_TUN_MODULE)
 struct socket *tun_get_socket(struct file *);
 struct ptr_ring *tun_get_tx_ring(struct file *file);
-- 
2.34.1

[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 <jasowang@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

Re: [PATCH net-next 1/2] tun: remove unnecessary tun_xdp_hdr structure

[Willem de Bruijn]

Jason Wang wrote:
> With f95f0f95cfb7("net, xdp: Introduce xdp_init_buff utility routine"),
> buffer length could be stored as frame size so there's no need to have
> a dedicated tun_xdp_hdr structure. We can simply store virtio net
> header instead.
> 
> Signed-off-by: Jason Wang <jasowang@redhat.com>

Acked-by: Willem de Bruijn <willemb@google.com>

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

[Willem de Bruijn]

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 <jasowang@redhat.com>

Acked-by: Willem de Bruijn <willemb@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
> 

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

[Jason Wang]

On Fri, Jun 13, 2025 at 10:16 AM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> 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 <jasowang@redhat.com>
>
> Acked-by: Willem de Bruijn <willemb@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.

We had this:

int pad = SKB_DATA_ALIGN(VHOST_NET_RX_PAD + headroom + nvq->sock_hlen);
int sock_hlen = nvq->sock_hlen;

So pad - sock_len is guaranteed to be greater than zero.

If this is not obvious, I can add a comment in the next version.

Thanks

>
> >       xdp_init_buff(xdp, buflen, NULL);
> >       xdp_prepare_buff(xdp, buf, pad, len, true);
> > --
> > 2.34.1
> >
>
>

Re: [PATCH net-next 1/2] tun: remove unnecessary tun_xdp_hdr structure

[Jason Wang]

On Thu, Jun 12, 2025 at 4:32 PM Jason Wang <jasowang@redhat.com> wrote:
>
> With f95f0f95cfb7("net, xdp: Introduce xdp_init_buff utility routine"),
> buffer length could be stored as frame size so there's no need to have
> a dedicated tun_xdp_hdr structure. We can simply store virtio net
> header instead.
>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---

Hello maintainers:

Are we ok with this series?

Thanks

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

[Paolo Abeni]

On 6/16/25 5:01 AM, Jason Wang wrote:
> On Fri, Jun 13, 2025 at 10:16 AM Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
>>
>> 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 <jasowang@redhat.com>
>>
>> Acked-by: Willem de Bruijn <willemb@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.
> 
> We had this:
> 
> int pad = SKB_DATA_ALIGN(VHOST_NET_RX_PAD + headroom + nvq->sock_hlen);
> int sock_hlen = nvq->sock_hlen;
> 
> So pad - sock_len is guaranteed to be greater than zero.
> 
> If this is not obvious, I can add a comment in the next version.

The relevant initializations are not visible in the patch itself, so I
think either a comment in the code or in the commit message would be useful.

Thanks,

Paolo