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

[David Ahern <dsahern@kernel.org>]

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

> 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.

> 
> 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.