netdev - Re: [PATCHv2 net] i40e: Implement ndo_gso

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Date:	Mon, 1 Dec 2014 16:09:23 -0800
From:	Tom Herbert <therbert@...gle.com>
To:	Jesse Gross <jesse@...ira.com>
Cc:	Joe Stringer <joestringer@...ira.com>,
	netdev <netdev@...r.kernel.org>,
	Shannon Nelson <shannon.nelson@...el.com>,
	"Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	"linux.nics" <linux.nics@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv2 net] i40e: Implement ndo_gso_check()

On Mon, Dec 1, 2014 at 3:53 PM, Jesse Gross <jesse@...ira.com> wrote:
> On Mon, Dec 1, 2014 at 3:47 PM, Tom Herbert <therbert@...gle.com> wrote:
>> On Mon, Dec 1, 2014 at 3:35 PM, Joe Stringer <joestringer@...ira.com> wrote:
>>> On 21 November 2014 at 09:59, Joe Stringer <joestringer@...ira.com> wrote:
>>>> On 20 November 2014 16:19, Jesse Gross <jesse@...ira.com> wrote:
>>>>> I don't know if we need to have the check at all for IPIP though -
>>>>> after all the driver doesn't expose support for it all (actually it
>>>>> doesn't expose GRE either). This raises kind of an interesting
>>>>> question about the checks though - it's pretty easy to add support to
>>>>> the driver for a new GSO type (and I imagine that people will be
>>>>> adding GRE soon) and forget to update the check.
>>>>
>>>> If the check is more conservative, then testing would show that it's
>>>> not working and lead people to figure out why (and update the check).
>>>
>>> More concretely, one suggestion would be something like following at
>>> the start of each gso_check():
>>>
>>> +       const int supported = SKB_GSO_TCPV4 | SKB_GSO_TCPV6 | SKB_GSO_FCOE |
>>> +                             SKB_GSO_UDP | SKB_GSO_UDP_TUNNEL;
>>> +
>>> +       if (skb_shinfo(skb)->gso_type & ~supported)
>>> +               return false;
>>
>> This should already be handled by net_gso_ok.
>
> My original point wasn't so much that this isn't handled at the moment
> but that it's easy to add a supported GSO type but then forget to
> update this check - i.e. if a driver already supports UDP_TUNNEL and
> adds support for GRE with the same constraints. It seems not entirely
> ideal that this function is acting as a blacklist rather than a
> whitelist.

Agreed, it would be nice to have all the checking logic in one place.
If all the drivers end up implementing ndo_gso_check then we could
potentially get rid of the GSO types as features. This probably
wouldn't be a bad thing since we already know that the features
mechanism doesn't scale (for instance there's no way to indicate that
certain combinations of GSO types are supported by a device).
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