[xdp-cloud] Re: Questions about Offloads and XDP-Hints regarding a Cloud-Provider Use-Case

[Jason Wang <jasowang@redhat.com>]

On Thu, Sep 29, 2022 at 6:37 AM Lorenzo Bianconi
<lorenzo.bianconi@redhat.com> wrote:
>
> > On 9/28/22 11:07 AM, Jesper Dangaard Brouer wrote:
> > >
> > > On 28/09/2022 15.54, Marcus Wichelmann wrote:
> > >>
> > >> I'm working for a cloud hosting provider and we're working on a new
> > >> XDP-based networking stack for our VM-Hosts that uses XDP to
> > >> accelerate the connectivity of our qemu/KVM VMs to the outside.
> > >>
> > >
> > > Welcome to the community! Sounds like an excellent use-case and
> > > opportunity for speeding up the RX packets from physical NIC into the
> > > VM.  Good to hear someone (again) having this use-case. I've personally
> >
> > +1

+1. This is somehow what I wanted to achieve in the past.

> >
> > > not been focused on this use-case lately, mostly because community
> > > members that I was interacting with changed jobs, away from cloud
> > > hosting companies. Good to have a user back in this area!
> > >
> > >
> > >> For this, we use XDP_REDIRECT to forward packets between the physical
> > >> host NIC and the VM tap devices. The main issue we have now is, that
> > >> our VM guests have some virtio NIC offloads enabled: rx/tx
> > >> checksumming, TSO/GSO, GRO and Scatter-Gather.
> > >
> > > Supporting RX-checksumming is part of the plans for XDP-hints, although
> > > virtio_net is not part of my initial patchset.
> >
> > Lorenzo and I both had versions of a patch to propagate rx csum
> > validation to the VM on xdp redirect. I do not recall a version after
> > this one:
> >
> > https://lore.kernel.org/netdev/cover.1622222367.git.lorenzo@kernel.org/
> >
> > and I lost of track of what change is needed for it to go in.
>
> iirc the blocking point was the missing CHECKSUM_COMPLETE support (we support
> just CHECKSUM_UNNECESSARY). We can conver it with XDP hw-hints.

How about other metadata like gso_type, gso_segs, csum_start and
csum_offset etc? This looks like a must for GSO to work even if
multi-buffer is supported.

Thanks

>
> Regards,
> Lorenzo
>
> >
> > >
> > > XDP-redirect with GRO and Scatter-Gather frames are part of the
> > > multi-buff effort (Cc Lorenzo), but currently XDP_REDIRECT with
> > > multi-buff is disabled (except for cpumap), because the lack of
> > > XDP-feature bits, meaning we cannot determine (in kernel) if receiving
> > > net_device supports multi-buff (Cc Kumar).
> > >
> > >> Currently, these offloads (especially TSO/GSO) are incompatible with
> > >> XDP_REDIRECT and result in packets being dropped. Because disabling
> > >> these offloads in all our customer VMs is not a good option, we're
> > >> searching for ways to support these offloads with XDP.
> > >>
> > >
> > > To David Ahern, didn't the kernel recently loosen up on having to
> > > disable these offloads for KVM virtio_net?
> >
> > not that I am aware. Still need tx offloads disabled.
> >
> >
> > This summarizes what I was looking into back in 2020, along with the
> > current state of XDP for VM use case:
> >
> > https://legacy.netdevconf.info/0x14/pub/slides/24/netdev-0x14-XDP-and-the-cloud.pdf
> >
> > source code is still on github too.
> >