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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 17 Feb 2023 08:47:36 -0500

On Fri, Feb 17, 2023 at 01:53:55PM +0100, Jiri Pirko wrote:
> Fri, Feb 17, 2023 at 01:22:01PM CET, [email-masked] wrote:
> >On Fri, Feb 17, 2023 at 01:15:47PM +0100, Jiri Pirko wrote:
> >> From: Jiri Pirko <[email-masked]>
> >> 
> >> 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 <[email-masked]>
> >
> >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.

> 
> >
> >thanks!
> >
> >
> >> ---
> >>  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
> >