Subject: Re: [patch net-next] net: virtio_net: implement exact header length guest feature

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 17 Feb 2023 07:22:01 -0500

On Fri, Feb 17, 2023 at 01:15:47PM +0100, Jiri Pirko wrote:
> From: Jiri Pirko <[email-masked]>
> 
> virtio_net_hdr_from_skb() fills up hdr_len to skb_headlen(skb).
> 
> Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN which when
> set implicates that the driver provides the exact size of the header.
> 
> The driver already complies to fill the correct value. Introduce the
> feature and advertise it.
> 
> Signed-off-by: Jiri Pirko <[email-masked]>

Could you add a bit of motivation just for the record?
Does this improve performance for some card? By how much?
Expected to help some future card?

thanks!

> ---
>  drivers/net/virtio_net.c        | 6 ++++--
>  include/uapi/linux/virtio_net.h | 1 +
>  2 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index fb5e68ed3ec2..e85b03988733 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -62,7 +62,8 @@ static const unsigned long guest_offloads[] = {
>  	VIRTIO_NET_F_GUEST_UFO,
>  	VIRTIO_NET_F_GUEST_CSUM,
>  	VIRTIO_NET_F_GUEST_USO4,
> -	VIRTIO_NET_F_GUEST_USO6
> +	VIRTIO_NET_F_GUEST_USO6,
> +	VIRTIO_NET_F_GUEST_HDRLEN
>  };
>  
>  #define GUEST_OFFLOAD_GRO_HW_MASK ((1ULL << VIRTIO_NET_F_GUEST_TSO4) | \
> @@ -4213,7 +4214,8 @@ static struct virtio_device_id id_table[] = {
>  	VIRTIO_NET_F_CTRL_MAC_ADDR, \
>  	VIRTIO_NET_F_MTU, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS, \
>  	VIRTIO_NET_F_SPEED_DUPLEX, VIRTIO_NET_F_STANDBY, \
> -	VIRTIO_NET_F_RSS, VIRTIO_NET_F_HASH_REPORT, VIRTIO_NET_F_NOTF_COAL
> +	VIRTIO_NET_F_RSS, VIRTIO_NET_F_HASH_REPORT, VIRTIO_NET_F_NOTF_COAL, \
> +	VIRTIO_NET_F_GUEST_HDRLEN
>  
>  static unsigned int features[] = {
>  	VIRTNET_FEATURES,
> diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
> index b4062bed186a..12c1c9699935 100644
> --- a/include/uapi/linux/virtio_net.h
> +++ b/include/uapi/linux/virtio_net.h
> @@ -61,6 +61,7 @@
>  #define VIRTIO_NET_F_GUEST_USO6	55	/* Guest can handle USOv6 in. */
>  #define VIRTIO_NET_F_HOST_USO	56	/* Host can handle USO in. */
>  #define VIRTIO_NET_F_HASH_REPORT  57	/* Supports hash report */
> +#define VIRTIO_NET_F_GUEST_HDRLEN  59	/* Guest provides the exact hdr_len value. */
>  #define VIRTIO_NET_F_RSS	  60	/* Supports RSS RX steering */
>  #define VIRTIO_NET_F_RSC_EXT	  61	/* extended coalescing info */
>  #define VIRTIO_NET_F_STANDBY	  62	/* Act as standby for another device
> -- 
> 2.39.0