RE: [patch net-next v2] net: virtio_net: implement exact header length guest feature

[Parav Pandit via Virtualization]

> From: Jiri Pirko <j...@resnulli.us>
> Sent: Tuesday, February 21, 2023 9:48 AM
> 
> From: Jiri Pirko <j...@nvidia.com>
> 
> Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN which when
> set implicates that the driver provides the exact size of the header.
> 
> 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.
> 
> Signed-off-by: Jiri Pirko <j...@nvidia.com>

This also saves a few cycles of L2 to L4 header parsing on every GSO stream for 
virtio PCI PF and VF devices that we have.

> ---
> 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

Reviewed-by: Parav Pandit <pa...@nvidia.com>
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