Re: [patch net-next v3] net: virtio_net: implement exact header length guest feature

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Feb 22, 2023 at 09:06:38AM +0100, Jiri Pirko wrote:
> From: Jiri Pirko <jiri@nvidia.com>
> 
> Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN which when
> which when 

which when which when is probably unintentional :)

>set implicates that device benefits from knowing the exact
> size of the header. For compatibility, to signal to the device that
> the header is reliable driver also needs to set this feature.
> Without this feature set by driver, device has to figure
> out the header size itself.
> 
> Quoting the original virtio spec:
> "hdr_len is a hint to the device as to how much of the header needs to
>  be kept to copy into each packet"
> 
> "a hint" might not be clear for the reader what does it mean, if it is
> "maybe like that" of "exactly like that". This feature just makes it
> crystal clear and let the device count on the hdr_len being filled up
> by the exact length of header.
> 
> Also note the spec already has following note about hdr_len:
> "Due to various bugs in implementations, this field is not useful
>  as a guarantee of the transport header size."
> 
> Without this feature the device needs to parse the header in core
> data path handling. Accurate information helps the device to eliminate
> such header parsing and directly use the hardware accelerators
> for GSO operation.
> 
> virtio_net_hdr_from_skb() fills up hdr_len to skb_headlen(skb).
> The driver already complies to fill the correct value. Introduce the
> feature and advertise it.
> 
> Note that virtio spec also includes following note for device
> implementation:
> "Caution should be taken by the implementation so as to prevent
>  a malicious driver from attacking the device by setting
>  an incorrect hdr_len."
> 
> There is a plan to support this feature in our emulated device.
> A device of SolidRun offers this feature bit. They claim this feature
> will save the device a few cycles for every GSO packet.
> 
> Link: https://docs.oasis-open.org/virtio/virtio/v1.2/cs01/virtio-v1.2-cs01.html#x1-230006x3
> Signed-off-by: Jiri Pirko <jiri@nvidia.com>
> Reviewed-by: Parav Pandit <parav@nvidia.com>
> Reviewed-by: Alvaro Karsz <alvaro.karsz@solid-run.com>
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> ---
> v2->v3:
> - changed the first paragraph in patch description according to
>   Michael's suggestion
> - added Link tag with link to the spec
> v1->v2:
> - extended patch description
> ---
>  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