netdev - Re: [RFC][net-next-2.6 PATCH v2] 8021q: set hard_header_len when VLAN offload features are toggled

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Message-ID: <AANLkTi=9tL5yrVGWsOagcgyKte5z8R9ADdz5n-Uf2Lsw@mail.gmail.com>
Date:	Tue, 26 Oct 2010 19:05:07 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	John Fastabend <john.r.fastabend@...el.com>
Cc:	netdev@...r.kernel.org, bhutchings@...arflare.com
Subject: Re: [RFC][net-next-2.6 PATCH v2] 8021q: set hard_header_len when VLAN
 offload features are toggled

On Tue, Oct 26, 2010 at 2:59 PM, John Fastabend
<john.r.fastabend@...el.com> wrote:
> Toggling the vlan tx|rx hw offloads needs to set the hard_header_len
> as well otherwise we end up using LL_RESERVED_SPACE incorrectly.
> This results in pskb_expand_head() being used unnecessarily.
>
> This add a check in vlan_transfer_features  to catch the ETH_FLAG_TXVLAN
> flag and set the header length. This requires drivers to add the
> ETH_FLAG_TXVLAN to vlan_features.
>
> Signed-off-by: John Fastabend <john.r.fastabend@...el.com>

I think this addresses all of the original problems.  However, I don't
think that we want to have drivers claim to support vlan offloading as
a feature for vlan packets.  That implies some type of QinQ
functionality to me.  In addition, if the vlan device claims to
support offloading and a second vlan device is stacked on top of it,
then the two will clobber skb->vlan_tci.  It's probably simpler to
just keep track of whether vlan offloading is currently enabled so we
can find out whether it changed.

> ---
>
>  net/8021q/vlan.c |   10 ++++++++++
>  1 files changed, 10 insertions(+), 0 deletions(-)
>
> diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c
> index 05b867e..825011b 100644
> --- a/net/8021q/vlan.c
> +++ b/net/8021q/vlan.c
> @@ -334,6 +334,16 @@ static void vlan_transfer_features(struct net_device *dev,
>        vlandev->features &= ~dev->vlan_features;
>        vlandev->features |= dev->features & dev->vlan_features;
>        vlandev->gso_max_size = dev->gso_max_size;
> +
> +       /* is ETH_FLAGS_TXVLAN being toggled */
> +       if ((vlandev->features & ETH_FLAG_TXVLAN) ^
> +           (old_features & ETH_FLAG_TXVLAN)) {
> +               if (vlandev->features & ETH_FLAG_TXVLAN)
> +                       vlandev->hard_header_len -= VLAN_HLEN;
> +               else
> +                       vlandev->hard_header_len += VLAN_HLEN;
> +       }

The correct flag for dev->features is NETIF_F_HW_VLAN_TX.
ETH_FLAGS_TXVLAN is an ethtool construct (that happens to have the
same value).

Thanks.
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