net: virtio_net: implement exact header length guest feature

Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN 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#x1-230006x3Signed-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>
Acked-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20230309094559.917857-1-jiri@resnulli.usSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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,

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