netdev - Re: tap/bridge: Dropping NETIF_F_GSO/NETIF_F

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Message-ID: <20110516081841.GA8073@redhat.com>
Date:	Mon, 16 May 2011 11:18:41 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Herbert Xu <herbert@...dor.hengli.com.au>
Cc:	Shan Wei <shanwei@...fujitsu.com>,
	Michał Mirosław <mirq-linux@...e.qmqm.pl>,
	netdev@...r.kernel.org, Ben Hutchings <bhutchings@...arflare.com>
Subject: Re: tap/bridge: Dropping NETIF_F_GSO/NETIF_F_SG

On Mon, May 16, 2011 at 06:07:02PM +1000, Herbert Xu wrote:
> On Mon, May 16, 2011 at 10:32:10AM +0300, Michael S. Tsirkin wrote:
> >
> > So how is NETIF_F_SG supposed to be enabled then?
> 
> It should either be enabled at device creation time, or whatever
> user-space entity managing the device creation should enable it
> along with checksumming and anything else applicable.

There's no interface for userspace to enable it: userspace
only has an ioctl to enable/disable checksum offloading.
SG is an implementation detail.

> > In upstream kernels userspace can disable checksum offloading then
> > re-enable and get SG set back.  userspace came to depend on this
> > behaviour so I think changing this is a regression.
> 
> Can you point me to the relevant code in the upstream kernel?
> I'm not aware of any automatic SG enabling for network devices
> in general when you enable checksum offloading.
> 
> Cheers,

I think what happens _SG is enabled at device creation time and
then upstream just keeps it on always, even when user clears
CSUM. With net-next code changed so that _SG gets cleared when CSUM
'gets cleared. But then it does not get reenabled when CSUM
gets reenabled.

> -- 
> Email: Herbert Xu <herbert@...dor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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