Date:   Tue, 21 Feb 2023 10:24:21 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org,
        davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        edumazet@...gle.com, virtualization@...ts.linux-foundation.org,
        Vitaly Mireyno <vmireyno@...vell.com>
Subject: Re: [patch net-next] net: virtio_net: implement exact header length
 guest feature

On Mon, Feb 20, 2023 at 8:55 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Mon, Feb 20, 2023 at 09:35:00AM +0100, Jiri Pirko wrote:
> > Fri, Feb 17, 2023 at 02:47:36PM CET, mst@...hat.com wrote:
> > >On Fri, Feb 17, 2023 at 01:53:55PM +0100, Jiri Pirko wrote:
> > >> Fri, Feb 17, 2023 at 01:22:01PM CET, mst@...hat.com wrote:
> > >> >On Fri, Feb 17, 2023 at 01:15:47PM +0100, Jiri Pirko wrote:
> > >> >> From: Jiri Pirko <jiri@...dia.com>
> > >> >>
> > >> >> 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 <jiri@...dia.com>
> > >> >
> > >> >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 ...

It should work otherwise it's a bug after this commit:

commit 96f8d9ecf227638c89f98ccdcdd50b569891976c
Author: Jason Wang <jasowang@...hat.com>
Date:   Wed Nov 13 14:00:39 2013 +0800

    tuntap: limit head length of skb allocated

Thanks

>
> > 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
> > >> >
> > >
>

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