Aw: Re: [PATCH 2/2] net: dsa: tag_ksz: dont let the hardware process the layer 4 checksum

[Lino Sanfilippo]

Hi Vladimir,

> Gesendet: Donnerstag, 15. Juli 2021 um 08:54 Uhr
> Von: "Vladimir Oltean" <olteanv@xxxxxxxxx>
> An: "Andrew Lunn" <andrew@xxxxxxx>
> Cc: "Lino Sanfilippo" <LinoSanfilippo@xxxxxx>, woojung.huh@xxxxxxxxxxxxx, UNGLinuxDriver@xxxxxxxxxxxxx, vivien.didelot@xxxxxxxxx, f.fainelli@xxxxxxxxx, davem@xxxxxxxxxxxxx, kuba@xxxxxxxxxx, netdev@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
> Betreff: Re: [PATCH 2/2] net: dsa: tag_ksz: dont let the hardware process the layer 4 checksum
>
> On Wed, Jul 14, 2021 at 10:15:20PM +0200, Andrew Lunn wrote:
> > On Wed, Jul 14, 2021 at 10:48:12PM +0300, Vladimir Oltean wrote:
> > > Hi Lino,
> > >
> > > On Wed, Jul 14, 2021 at 09:17:23PM +0200, Lino Sanfilippo wrote:
> > > > If the checksum calculation is offloaded to the network device (e.g due to
> > > > NETIF_F_HW_CSUM inherited from the DSA master device), the calculated
> > > > layer 4 checksum is incorrect. This is since the DSA tag which is placed
> > > > after the layer 4 data is seen as a part of the data portion and thus
> > > > errorneously included into the checksum calculation.
> > > > To avoid this, always calculate the layer 4 checksum in software.
> > > >
> > > > Signed-off-by: Lino Sanfilippo <LinoSanfilippo@xxxxxx>
> > > > ---
> > >
> > > This needs to be solved more generically for all tail taggers. Let me
> > > try out a few things tomorrow and come with a proposal.
> >
> > Maybe the skb_linearize() is also a generic problem, since many of the
> > tag drivers are using skb_put()? It looks like skb_linearize() is
> > cheap because checking if the skb is already linear is cheap. So maybe
> > we want to do it unconditionally?
>
> Yeah, but we should let the stack deal with both issues in validate_xmit_skb().
> There is a skb_needs_linearize() call which returns false because the
> DSA interface inherits NETIF_F_SG from the master via dsa_slave_create():
>
> 	slave_dev->features = master->vlan_features | NETIF_F_HW_TC;
>
> Arguably that's the problem right there, we shouldn't inherit neither
> NETIF_F_HW_CSUM nor NETIF_F_SG from the list of features inheritable by
> 8021q uppers.
>
> - If we inherit NETIF_F_SG we obligate ourselves to call skb_linearize()
>   for tail taggers on xmit. DSA probably doesn't break anything for
>   header taggers though even if the xmit skb is paged, since the DSA
>   header would always be part of the skb head, so this is a feature we
>   could keep for them.
> - If we inherit NETIF_F_HW_CSUM from the master for tail taggers, it is
>   actively detrimential to keep this feature enabled, as proven my Lino.
>   As for header taggers, I fail to see how this would be helpful, since
>   the DSA master would always fail to see the real IP header (it has
>   been pushed to the right by the DSA tag), and therefore, the DSA
>   master offload would be effectively bypassed. So no point, really, in
>   inheriting it in the first place in any situation.
>
> Lino, to fix these bugs by letting validate_xmit_skb() know what works
> for DSA and what doesn't, could you please:
>
> (a) move the current slave_dev->features assignment to
>     dsa_slave_setup_tagger()? We now support changing the tagging
>     protocol at runtime, and everything that depends on what the tagging
>     protocol is (in this case, tag_ops->needed_headroom vs
>     tag_ops->needed_tailroom) should be put in that function.
> (b) unconditionally clear NETIF_F_HW_CSUM from slave_dev->features,
>     after inheriting the vlan_features from the master?
> (c) clear NETIF_F_SG from slave_dev->features if we have a non-zero
>     tag_ops->needed_tailroom?
>

Sure, I will test this solution. But I think NETIF_F_FRAGLIST should also be
cleared in this case, right?


Regards,
Lino