Re: [PATCH] net: add initial support for AF_XDP network backend

[Jason Wang]

On Fri, Jun 23, 2023 at 5:58 AM Ilya Maximets <i.maximets@ovn.org> wrote:
>
> AF_XDP is a network socket family that allows communication directly
> with the network device driver in the kernel, bypassing most or all
> of the kernel networking stack.  In the essence, the technology is
> pretty similar to netmap.  But, unlike netmap, AF_XDP is Linux-native
> and works with any network interfaces without driver modifications.
> Unlike vhost-based backends (kernel, user, vdpa), AF_XDP doesn't
> require access to character devices or unix sockets.  Only access to
> the network interface itself is necessary.
>
> This patch implements a network backend that communicates with the
> kernel by creating an AF_XDP socket.  A chunk of userspace memory
> is shared between QEMU and the host kernel.  4 ring buffers (Tx, Rx,
> Fill and Completion) are placed in that memory along with a pool of
> memory buffers for the packet data.  Data transmission is done by
> allocating one of the buffers, copying packet data into it and
> placing the pointer into Tx ring.  After transmission, device will
> return the buffer via Completion ring.  On Rx, device will take
> a buffer form a pre-populated Fill ring, write the packet data into
> it and place the buffer into Rx ring.
>
> AF_XDP network backend takes on the communication with the host
> kernel and the network interface and forwards packets to/from the
> peer device in QEMU.
>
> Usage example:
>
>   -device virtio-net-pci,netdev=guest1,mac=00:16:35:AF:AA:5C
>   -netdev af-xdp,ifname=ens6f1np1,id=guest1,mode=native,queues=1
>
> XDP program bridges the socket with a network interface.  It can be
> attached to the interface in 2 different modes:
>
> 1. skb - this mode should work for any interface and doesn't require
>          driver support.  With a caveat of lower performance.
>
> 2. native - this does require support from the driver and allows to
>             bypass skb allocation in the kernel and potentially use
>             zero-copy while getting packets in/out userspace.
>
> By default, QEMU will try to use native mode and fall back to skb.
> Mode can be forced via 'mode' option.  To force 'copy' even in native
> mode, use 'force-copy=on' option.  This might be useful if there is
> some issue with the driver.
>
> Option 'queues=N' allows to specify how many device queues should
> be open.  Note that all the queues that are not open are still
> functional and can receive traffic, but it will not be delivered to
> QEMU.  So, the number of device queues should generally match the
> QEMU configuration, unless the device is shared with something
> else and the traffic re-direction to appropriate queues is correctly
> configured on a device level (e.g. with ethtool -N).
> 'start-queue=M' option can be used to specify from which queue id
> QEMU should start configuring 'N' queues.  It might also be necessary
> to use this option with certain NICs, e.g. MLX5 NICs.  See the docs
> for examples.
>
> In a general case QEMU will need CAP_NET_ADMIN and CAP_SYS_ADMIN
> capabilities in order to load default XSK/XDP programs to the
> network interface and configure BTF maps.

I think you mean "BPF" actually?

>  It is possible, however,
> to run only with CAP_NET_RAW.

Qemu often runs without any privileges, so we need to fix it.

I think adding support for SCM_RIGHTS via monitor would be a way to go.


> For that to work, an external process
> with admin capabilities will need to pre-load default XSK program
> and pass an open file descriptor for this program's 'xsks_map' to
> QEMU process on startup.  Network backend will need to be configured
> with 'inhibit=on' to avoid loading of the programs.  The file
> descriptor for 'xsks_map' can be passed via 'xsks-map-fd=N' option.
>
> There are few performance challenges with the current network backends.
>
> First is that they do not support IO threads.

The current networking codes needs some major recatoring to support IO
threads which I'm not sure is worthwhile.

> This means that data
> path is handled by the main thread in QEMU and may slow down other
> work or may be slowed down by some other work.  This also means that
> taking advantage of multi-queue is generally not possible today.
>
> Another thing is that data path is going through the device emulation
> code, which is not really optimized for performance.  The fastest
> "frontend" device is virtio-net.  But it's not optimized for heavy
> traffic either, because it expects such use-cases to be handled via
> some implementation of vhost (user, kernel, vdpa).  In practice, we
> have virtio notifications and rcu lock/unlock on a per-packet basis
> and not very efficient accesses to the guest memory.  Communication
> channels between backend and frontend devices do not allow passing
> more than one packet at a time as well.
>
> Some of these challenges can be avoided in the future by adding better
> batching into device emulation or by implementing vhost-af-xdp variant.

It might require you to register(pin) the whole guest memory to XSK or
there could be a copy. Both of them are sub-optimal.

A really interesting project is to do AF_XDP passthrough, then we
don't need to care about pin and copy and we will get ultra speed in
the guest. (But again, it might needs BPF support in virtio-net).

>
> There are also a few kernel limitations.  AF_XDP sockets do not
> support any kinds of checksum or segmentation offloading.  Buffers
> are limited to a page size (4K), i.e. MTU is limited.  Multi-buffer
> support is not implemented for AF_XDP today.  Also, transmission in
> all non-zero-copy modes is synchronous, i.e. done in a syscall.
> That doesn't allow high packet rates on virtual interfaces.
>
> However, keeping in mind all of these challenges, current implementation
> of the AF_XDP backend shows a decent performance while running on top
> of a physical NIC with zero-copy support.
>
> Test setup:
>
> 2 VMs running on 2 physical hosts connected via ConnectX6-Dx card.
> Network backend is configured to open the NIC directly in native mode.
> The driver supports zero-copy.  NIC is configured to use 1 queue.
>
> Inside a VM - iperf3 for basic TCP performance testing and dpdk-testpmd
> for PPS testing.
>
> iperf3 result:
>  TCP stream      : 19.1 Gbps
>
> dpdk-testpmd (single queue, single CPU core, 64 B packets) results:
>  Tx only         : 3.4 Mpps
>  Rx only         : 2.0 Mpps
>  L2 FWD Loopback : 1.5 Mpps

I don't object to merging this backend (considering we've already
merged netmap) once the code is fine, but the number is not amazing so
I wonder what is the use case for this backend?

Thanks