On Fri, Feb 17, 2023 at 01:53:55PM +0100, Jiri Pirko wrote:

Fri, Feb 17, 2023 at 01:22:01PM CET, mst@redhat.com wrote:

quoted

On Fri, Feb 17, 2023 at 01:15:47PM +0100, Jiri Pirko wrote:

quoted

From: Jiri Pirko <redacted>

virtio_net_hdr_from_skb() fills up hdr_len to skb_headlen(skb).

Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN which when
set implicates that the driver provides the exact size of the header.

The driver already complies to fill the correct value. Introduce the
feature and advertise it.

Signed-off-by: Jiri Pirko <redacted>

Could you add a bit of motivation just for the record?
Does this improve performance for some card? By how much?
Expected to help some future card?

I can get that info, but isn't that rather something to be appended to
the virtio-spec patch? I mean, the feature is there, this is just
implementing it in one driver.

It is more like using it in the driver.  It's not like we have to use
everything - it could be useful for e.g. dpdk but not linux.
Implementing it in the Linux driver has support costs - for example what
if there's a bug and sometimes the length is incorrect?
We'll be breaking things.

The patch was submitted by Marvell but they never bothered with
using it in Linux. I guess they are using it for something else?
CC Vitaly who put this in.

quoted

thanks!

quoted

---
 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