netdev - RE: [PATCH net] net: Generalize ndo_gso_check to ndo_features

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Date:	Tue, 23 Dec 2014 06:24:48 +0000
From:	Sathya Perla <Sathya.Perla@...lex.Com>
To:	Tom Herbert <therbert@...gle.com>, Jesse Gross <jesse@...ira.com>
CC:	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Joe Stringer <joestringer@...ira.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: RE: [PATCH net] net: Generalize ndo_gso_check to ndo_features_check

> -----Original Message-----
> From: netdev-owner@...r.kernel.org [mailto:netdev-
> owner@...r.kernel.org] On Behalf Of Tom Herbert
> 
> On Mon, Dec 22, 2014 at 8:03 AM, Jesse Gross <jesse@...ira.com> wrote:
> > GSO isn't the only offload feature with restrictions that
> > potentially can't be expressed with the current features mechanism.
> > Checksum is another although it's a general issue that could in
> > theory apply to anything. Even if it may be possible to
> > implement these restrictions in other ways, it can result in
> > duplicate code or inefficient per-packet behavior.
> >
> > This generalizes ndo_gso_check so that drivers can remove any
> > features that don't make sense for a given packet, similar to
> > netif_skb_features(). It also converts existing driver
> > restrictions to the new format, completing the work that was
> > done to support tunnel protocols since the issues apply to
> > checksums as well.
> >
> It's a nice feature, but I really hope that this is not used for
> checksums. We already have a sufficiently general interface for that
> and checksum is already computed in drivers to work around HW bugs.
> 
The ndo_featureas_check() interface that includes a means to report
inability to compute inner checksums on some tunnel types, seems like
a useful feature. The Skyhawk-R NIC can support inner csum offload for
either vxlan or nv-gre, but not both simultaneously. So, this ndo_
is useful in reporting this situation.
This would also obviate the need to have extra code in the drivers to
compute csums in the above scenario.

thanks,
-Sathya