netdev - Re: [PATCH net-next] virtio-net: ethtool configurable LRO

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Date:   Thu, 20 Dec 2018 13:48:58 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jason Wang <jasowang@...hat.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next] virtio-net: ethtool configurable LRO

On Thu, Dec 20, 2018 at 01:30:15PM -0500, Willem de Bruijn wrote:
> On Wed, Dec 19, 2018 at 5:44 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > On Wed, Dec 19, 2018 at 5:32 PM Willem de Bruijn
> > <willemdebruijn.kernel@...il.com> wrote:
> > >
> > > From: Willem de Bruijn <willemb@...gle.com>
> > >
> > > Virtio-net devices negotiate LRO support with the host.
> > > Display the initially negotiated state with ethtool -k.
> > >
> > > Also allow configuring it with ethtool -K, reusing the existing
> > > virtnet_set_guest_offloads helper that disables LRO for XDP.
> > >
> > > Virtio-net negotiates TSO4 and TSO6 separately, but ethtool does not
> > > distinguish between the two. In the unlikely legacy case where the
> > > host only supports one of the two, maintain the current state.
> > >
> > > RTNL is held while calling virtnet_set_features, same as on the path
> > > from virtnet_xdp_set.
> > >
> > > Tested:
> > >   Verified that large-receive-offload is "off [fixed]" when not
> > >   negotiated during initial probe and "on" otherwise.
> > >
> > >   Measured packet size when scp-ing a large file to the guest with
> > >   nstat (IpExtInOctets / IpInDelivers). Verified that packet size
> > >   exceeds MTU only if ethtool -k shows LRO and/or GRO as enabled.
> > >
> > > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> >
> > Two comments for the review:
> >
> > 1. Current drivers show LRO as disabled even if it is negotiated at
> > probe. That is arguably a bug. In which case I can split this patch into
> > - a net patch to advertise the initially negotiated value through dev->feature
> > - a net-next patch to configure it with ndo_set_features.
> >
> > 2. virtnet_clear_guest_offloads disables all guest_offloads, including
> > VIRTIO_NET_F_GUEST_CSUM. That seems undesirable? The feature is a
> > prerequisite for LRO, but not the other way around. This applies
> > equally to the xdp and ethtool paths, so would be another  separate
> > net patch.
> 
> I was wrong. This was a deliberate choice. See below. Will leave as is.
> 
> The other change is not really important enough on its, so I'll just
> send a v2 of this patch.
> 
> One additional change, show LRO as enabled if it is active for any protocol
> (as opposed to only for all protocols), so that we can truly disable it in all
> cases.

That makes sense.

> "
> commit e59ff2c49ae16e1d179de679aca81405829aee6c
> Author: Jason Wang <jasowang@...hat.com>
> Date:   Thu Nov 22 14:36:30 2018 +0800
> 
>     virtio-net: disable guest csum during XDP set
> 
>     We don't disable VIRTIO_NET_F_GUEST_CSUM if XDP was set. This means we
>     can receive partial csumed packets with metadata kept in the
>     vnet_hdr. This may have several side effects:
> 
>     - It could be overridden by header adjustment, thus is might be not
>       correct after XDP processing.
>     - There's no way to pass such metadata information through
>       XDP_REDIRECT to another driver.
>     - XDP does not support checksum offload right now.
> 
>     So simply disable guest csum if possible in this the case of XDP.
> "