Virtualization - Feb 2023 - [patch net-next v3] net: virtio_net: implement exact header length guest feature

From: Jiri Pirko <jiri at nvidia.com>

Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN which when
which when 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 at nvidia.com>
Reviewed-by: Parav Pandit <parav at nvidia.com>
Reviewed-by: Alvaro Karsz <alvaro.karsz at solid-run.com>
Acked-by: Michael S. Tsirkin <mst at 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

On Wed, Feb 22, 2023 at 09:06:38AM +0100, Jiri Pirko
wrote:
> From: Jiri Pirko <jiri at 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 at nvidia.com>
> Reviewed-by: Parav Pandit <parav at nvidia.com>
> Reviewed-by: Alvaro Karsz <alvaro.karsz at solid-run.com>
> Acked-by: Michael S. Tsirkin <mst at 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