netdev - Re: [PATCH net-next] gso: enable udp gso for virtual devices

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Date:   Thu, 13 Jun 2019 17:20:35 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jason Baron <jbaron@...mai.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>, Joshua Hunt <johunt@...mai.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next] gso: enable udp gso for virtual devices

> >> @@ -237,6 +237,7 @@ static inline int find_next_netdev_feature(u64 feature, unsigned long start)
> >>                                  NETIF_F_GSO_GRE_CSUM |                 \
> >>                                  NETIF_F_GSO_IPXIP4 |                   \
> >>                                  NETIF_F_GSO_IPXIP6 |                   \
> >> +                                NETIF_F_GSO_UDP_L4 |                   \
> >>                                  NETIF_F_GSO_UDP_TUNNEL |               \
> >>                                  NETIF_F_GSO_UDP_TUNNEL_CSUM)
> >
> > Are you adding this to NETIF_F_GSO_ENCAP_ALL? Wouldn't it make more
> > sense to add it to NETIF_F_GSO_SOFTWARE?
> >
>
> Yes, I'm adding to NETIF_F_GSO_ENCAP_ALL (not very clear from the
> context). I will fix the commit log.
>
> In: 83aa025 udp: add gso support to virtual devices, the support was
> also added to NETIF_F_GSO_ENCAP_ALL (although subsequently reverted due
> to UDP GRO not being in place), so I wonder what the reason was for that?

That was probably just a bad choice on my part.

It worked in practice, but if NETIF_F_GSO_SOFTWARE works the same
without unexpected side effects, then I agree that it is the better choice.

That choice does appear to change behavior when sending over tunnel
devices. Might it send tunneled GSO packets over loopback?



> I agree that NETIF_F_GSO_SOFTWARE seems conceptually more logical and
> further I think it adds support for more 'virtual' devices. For example,
> I tested loopback with NETIF_F_GSO_UDP_L4 being added to
> NETIF_F_GSO_SOFTWARE and it shows a nice performance gain, whereas
> NETIF_F_GSO_ENCAP_ALL isn't included for loopback.
>
> Thanks,
>
> -Jason