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![Ilya Maximets](/assets/img/avatar_default.png)
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 or CAP_BPF capabilities in order to load default XSK/XDP programs to the network interface and configure BPF maps. It is possible, however, to run with no capabilities. For that to work, an external process with enough capabilities will need to pre-load default XSK program, create AF_XDP sockets and pass their file descriptors to QEMU process on startup via 'sock-fds' option. Network backend will need to be configured with 'inhibit=on' to avoid loading of the program. QEMU will need 32 MB of locked memory (RLIMIT_MEMLOCK) per queue or CAP_IPC_LOCK. There are few performance challenges with the current network backends. First is that they do not support IO threads. 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. 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 implementation for AF_XDP is in progress, but not ready yet. 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 In skb mode the same setup shows much lower performance, similar to the setup where pair of physical NICs is replaced with veth pair: iperf3 result: TCP stream : 9 Gbps dpdk-testpmd (single queue, single CPU core, 64 B packets) results: Tx only : 1.2 Mpps Rx only : 1.0 Mpps L2 FWD Loopback : 0.7 Mpps Results in skb mode or over the veth are close to results of a tap backend with vhost=on and disabled segmentation offloading bridged with a NIC. Signed-off-by: Ilya Maximets <i.maximets@ovn.org> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com> (docker/lcitool) Signed-off-by: Jason Wang <jasowang@redhat.com>
89 lines
3.4 KiB
C
89 lines
3.4 KiB
C
/*
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* QEMU System Emulator
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*
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* Copyright (c) 2003-2008 Fabrice Bellard
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
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* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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#ifndef QEMU_NET_CLIENTS_H
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#define QEMU_NET_CLIENTS_H
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#include "net/net.h"
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int net_init_dump(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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#ifdef CONFIG_SLIRP
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int net_init_slirp(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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#endif
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int net_init_hubport(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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int net_init_socket(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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int net_init_stream(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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int net_init_dgram(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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int net_init_tap(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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int net_init_bridge(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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int net_init_l2tpv3(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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#ifdef CONFIG_VDE
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int net_init_vde(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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#endif
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#ifdef CONFIG_NETMAP
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int net_init_netmap(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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#endif
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#ifdef CONFIG_AF_XDP
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int net_init_af_xdp(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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#endif
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int net_init_vhost_user(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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int net_init_vhost_vdpa(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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#ifdef CONFIG_VMNET
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int net_init_vmnet_host(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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int net_init_vmnet_shared(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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int net_init_vmnet_bridged(const Netdev *netdev, const char *name,
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NetClientState *peer, Error **errp);
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#endif /* CONFIG_VMNET */
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#endif /* QEMU_NET_CLIENTS_H */
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