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XDP on Power

This post is a bit of a break from the standard IBM fare of this blog, as I now work for Canonical. But I have a soft spot for Power from my time at IBM - and Canonical officially supports 64-bit, little-endian Power - so when I get a spare moment I try to make sure that cool, officially-supported technologies work on Power before we end up with a customer emergency! So, without further ado, this is the story of XDP on Power.

XDP

eXpress Data Path (XDP) is a cool Linux technology to allow really fast processing of network packets.

Normally in Linux, a packet is received by the network card, an SKB (socket buffer) is allocated, and the packet is passed up through the networking stack.

This introduces an inescapable latency penalty: we have to allocate some memory and copy stuff around. XDP allows some network cards and drivers to process packets early - even before the allocation of the SKB. This is much faster, and so has applications in DDOS mitigation and other high-speed networking use-cases. The IOVisor project has much more information if you want to learn more.

eBPF

XDP processing is done by an eBPF program. eBPF - the extended Berkeley Packet Filter - is an in-kernel virtual machine with a limited set of instructions. The kernel can statically validate eBPF programs to ensure that they terminate and are memory safe. From this it follows that the programs cannot be Turing-complete: they do not have backward branches, so they cannot do fancy things like loops. Nonetheless, they're surprisingly powerful for packet processing and tracing. eBPF programs are translated into efficient machine code using in-kernel JIT compilers on many platforms, and interpreted on platforms that do not have a JIT. (Yes, there are multiple JIT implementations in the kernel. I find this a terrifying thought.)

Rather than requiring people to write raw eBPF programs, you can write them in a somewhat-restricted subset of C, and use Clang's eBPF target to translate them. This is super handy, as it gives you access to the kernel headers - which define a number of useful data structures like headers for various network protocols.

Trying it

There are a few really interesting project that are already up and running that allow you to explore XDP without learning the innards of both eBPF and the kernel networking stack. I explored the samples in the bcc compiler collection and also the samples from the netoptimizer/prototype-kernel repository.

The easiest way to get started with these is with a virtual machine, as recent virtio network drivers support XDP. If you are using Ubuntu, you can use the uvt-kvm tooling to trivially set up a VM running Ubuntu Zesty on your local machine.

Once your VM is installed, you need to shut it down and edit the virsh XML.

You need 2 vCPUs (or more) and a virtio+vhost network card. You also need to edit the 'interface' section and add the following snippet (with thanks to the xdp-newbies list):

<driver name='vhost' queues='4'>
    <host tso4='off' tso6='off' ecn='off' ufo='off'/>
    <guest tso4='off' tso6='off' ecn='off' ufo='off'/>
</driver>

(If you have more than 2 vCPUs, set the queues parameter to 2x the number of vCPUs.)

Then, install a modern clang (we've had issues with 3.8 - I recommend v4+), and the usual build tools.

I recommend testing with the prototype-kernel tools - the DDOS prevention tool is a good demo. Then - on x86 - you just follow their instructions. I'm not going to repeat that here.

POWERful XDP

What happens when you try this on Power? Regular readers of my posts will know to expect some minor hitches.

XDP does not disappoint.

Firstly, the prototype-kernel repository hard codes x86 as the architecture for kernel headers. You need to change it for powerpc.

Then, once you get the stuff compiled, and try to run it on a current-at-time-of-writing Zesty kernel, you'll hit a massive debug splat ending in:

32: (61) r1 = *(u32 *)(r8 +12)
misaligned packet access off 0+18+12 size 4
load_bpf_file: Permission denied

It turns out this is because in Ubuntu's Zesty kernel, CONFIG_HAS_EFFICIENT_UNALIGNED_ACCESS is not set on ppc64el. Because of that, the eBPF verifier will check that all loads are aligned - and this load (part of checking some packet header) is not, and so the verifier rejects the program. Unaligned access is not enabled because the Zesty kernel is being compiled for CPU_POWER7 instead of CPU_POWER8, and we don't have efficient unaligned access on POWER7.

As it turns out, IBM never released any officially supported Power7 LE systems - LE was only ever supported on Power8. So, I filed a bug and sent a patch to build Zesty kernels for POWER8 instead, and that has been accepted and will be part of the next stable update due real soon now.

Sure enough, if you install a kernel with that config change, you can verify the XDP program and load it into the kernel!

If you have real powerpc hardware, that's enough to use XDP on Power! Thanks to Michael Ellerman, maintainer extraordinaire, for verifying this for me.

If - like me - you don't have ready access to Power hardware, you're stuffed. You can't use qemu in TCG mode: to use XDP with a VM, you need multi-queue support, which only exists in the vhost driver, which is only available for KVM guests. Maybe IBM should release a developer workstation. (Hint, hint!)

Overall, I was pleasantly surprised by how easy things were for people with real ppc hardware - it's encouraging to see something not require kernel changes!

eBPF and XDP are definitely growing technologies - as Brendan Gregg notes, now is a good time to learn them! (And those on Power have no excuse either!)

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