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

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Date:   Tue, 21 Feb 2023 17:26:19 +0200
From:   Alvaro Karsz <alvaro.karsz@...id-run.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        pabeni@...hat.com, edumazet@...gle.com, mst@...hat.com,
        jasowang@...hat.com, virtualization@...ts.linux-foundation.org,
        vmireyno@...vell.com, parav@...dia.com
Subject: Re: [patch net-next v2] net: virtio_net: implement exact header
 length guest feature

>
> From: Jiri Pirko <jiri@...dia.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 <jiri@...dia.com>
> ---
> 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
>

This indeed saves some cycles in our device.
Thanks!

Reviewed-by: Alvaro Karsz <alvaro.karsz@...id-run.com>