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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 20 Feb 2023 07:55:33 -0500

On Mon, Feb 20, 2023 at 09:35:00AM +0100, Jiri Pirko wrote:
> Fri, Feb 17, 2023 at 02:47:36PM CET, [email-masked] wrote:
> >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.
> 
> I understand. To my understanding this feature just fixes the original
> ambiguity in the virtio spec.
> 
> 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.
> 
> If you look at the tap implementation, it uses hdr_len to alloc
> skb linear part. No hint, it counts with the provided value.
> So if the driver is currently not precise, it breaks tap.

Well that's only for gso though right?
And making it bigger than necessary works fine ...

> I will add this to the patch description and send v2.
> 

I feel this does not answer the question yet, or maybe I am being dense.
My point was not about making hdr_len precise.  My point was that we are
making a change here for no apparent reason. I am guessing you are not
doing it for fun - so why? Is there a device with this feature bit
you are aware of?

> 
> >
> >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
> >> >
> >