netdev - Re: Neterion and UFO handling [was: Re: [PATCH] ipv6: udp packets following an UFO enqueued packet need also be handled by UFO]

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Date:	Mon, 7 Oct 2013 18:53:43 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	jon.mason@...el.com
Cc:	jdmason@...zu.us, Jiri Pirko <jiri@...nulli.us>,
	netdev@...r.kernel.org, yoshfuji@...ux-ipv6.org,
	davem@...emloft.net, kuznet@....inr.ac.ru, jmorris@...ei.org,
	kaber@...sh.net, herbert@...dor.apana.org.au,
	eric.dumazet@...il.com
Subject: Re: Neterion and UFO handling [was: Re: [PATCH] ipv6: udp packets following an UFO enqueued packet need also be handled by UFO]

Hi Jon!

Maybe I got the wrong email address for the neterion driver from the
maintainers filer? If you are (still) affiliated with the neterion driver,
maybe you could have a short look at the quoted mail below?

Thanks,

  Hannes

On Wed, Oct 02, 2013 at 06:27:30PM +0200, Hannes Frederic Sowa wrote:
> Hi!
> 
> I have a question regarding UFO and the neterion driver, which as the only one
> advertises hardware UFO support:
> 
> The patch discusses in this thread
> http://thread.gmane.org/gmane.linux.network/284348/focus=285405 could change
> some semantics how packets are constructed before submitted to the driver.
> 
> We currently guarantee that we have the MAC/IP/UDP header in skb->data and the
> payload is attached in the skb's frags. With the changes discussed in this
> thread it is possible that we also append to skb->data some amount of data
> which is not targeted for the header. From reading the driver sources it seems
> the hardware interprets the skb->data to skb_headlen as the header, so we
> could include some data in the fragments more than once.
> 
> Do you think this change is safe? Otherwise I would suggest that the UFO
> capability is switched off until the driver signals the hardware the start and
> end of the headers correctly?
> 
> I left the mail below intact which points to the specific place in s2io.c
> where I think the problem is.
> 
> On Wed, Oct 02, 2013 at 08:14:27AM -0700, Eric Dumazet wrote:
> > On Wed, 2013-10-02 at 15:03 +0200, Hannes Frederic Sowa wrote:
> > > On Wed, Oct 02, 2013 at 02:12:07PM +0200, Hannes Frederic Sowa wrote:
> > > > Hi Eric!
> > > > 
> > > > On Wed, Oct 02, 2013 at 03:41:28AM -0700, Eric Dumazet wrote:
> > > > > On Wed, 2013-10-02 at 10:58 +0200, Jiri Pirko wrote:
> > > > > > Wed, Oct 02, 2013 at 01:25:34AM CEST, hannes@...essinduktion.org wrote:
> > > > > > >-	if (((length > mtu) || (skb && skb_is_gso(skb))) &&
> > > > > > >+	if (((length > mtu) || (skb && skb_has_frags(skb))) &&
> > > > > 
> > > > > > 
> > > > > > This seems correct to me. sk_is_gso would work as well is you apply my
> > > > > > patch "[patch net] ip6_output: do skb ufo init for peeked non ufo skb as
> > > > > > well" which does the setting of gso_size.
> > > > > 
> > > > > Well, skb having frags or not should not be a concern :
> > > > > Thats an allocation choice (lets say to avoid high order allocations). 
> > > > > 
> > > > > Setting gso_size is probably better.
> > > > 
> > > > e89e9cf539a28df7d0eb1d0a545368e9920b34ac ("[IPv4/IPv6]: UFO Scatter-gather
> > > > approach") states:
> > > > 
> > > > "
> > > > skb->data will contain MAC/IP/UDP header and skb_shinfo(skb)->frags[]
> > > > contains the data payload. The skb->ip_summed will be set to CHECKSUM_HW
> > > > indicating that hardware has to do checksum calculation. Hardware should
> > > > compute the UDP checksum of complete datagram and also ip header checksum of
> > > > each fragmented IP packet.
> > > > "
> > > > 
> > > > This is the reason why I tried not to update the gso_size. If it is ok, I am
> > > > fine with that.
> > > 
> > > Especially, drivers/net/ethernet/neterion/s2io.c states that the first dma
> > > mapping (skb->data with skb_headlen, which is fine) is used as the inband
> > > header:
> > > 
> > >         if (offload_type == SKB_GSO_UDP)
> > >                 frg_cnt++; /* as Txd0 was used for inband header */
> > > 
> > > That is my only other hint that we maybe should not update gso_size and
> > > gso_type. I guess software fallback does not have this problem, but I won't
> > > have time to check until this evening.
> > > 
> > > I am really not sure if just setting gso_size does not break neterion UFO
> > > offloading. :/
> > 
> > Well, just ask Jon Mason to double check ;)
> > 
> > I think the commit intent was to set gso_size :
> > 
> >    skb_shinfo(skb)->ufo_size will indicate the length of data part in each IP
> >     fragment going out of the adapter after IP fragmentation by hardware.
> > 
> > The fact that it states "skb->data will contain MAC/IP/UDP header and
> > skb_shinfo(skb)->frags[] contains the data payload." seems irrelevant.
> > 
> > If Neterion driver mandates that skb->head *only* contains the
> > MAC/IP/UDP header, that should be handled in the driver itself.
> 
> Thanks Eric for clearing this up.
> 
> I really thought it would be the common pattern for UFO to have only headers
> in skb->data, so I didn't bother to ask in the first place.
> 
> Thanks,
> 
>   Hannes
> 
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