> From: Jiri Pirko <jiri@xxxxxxxxxxx>
> Sent: Tuesday, February 21, 2023 9:48 AM
>
> From: Jiri Pirko <jiri@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
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 <parav@xxxxxxxxxx>
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