Note: This RFC has been sent to netdev as well as qemu-devel lists This series introduces XDP offloading from virtio_net. It is based on the following work by Jason Wang: https://netdevconf.info/0x13/session.html?xdp-offload-with-virtio-net Current XDP performance in virtio-net is far from what we can achieve on host. Several major factors cause the difference: - Cost of virtualization - Cost of virtio (populating virtqueue and context switching) - Cost of vhost, it needs more optimization - Cost of data copy Because of above reasons there is a need of offloading XDP program to host. This set is an attempt to implement XDP offload from the guest. * High level design: virtio_net exposes itself as offload capable device and works as a transport of commands to load the program on the host. When offload is requested, it sends the program to Qemu. Qemu then loads the program and attaches to corresponding tap device. Similarly virtio_net sends control commands to create and control maps. tap device runs the XDP prog in its Tx path. The fast datapath remains on host whereas slow path in which user program reads/updates map values remains in the guest. When offloading to actual hardware the program needs to be translated and JITed for the target hardware. In case of offloading from guest we pass almost raw program to the host. The verifier on the host verifies the offloaded program. * Implementation in Kernel virtio_net ========== Creates bpf offload device and registers as offload capable device. It also implements bpf_map_dev_ops to handle the offloaded map. A new command structure is defined to communicate with qemu. Map offload: - In offload sequence maps are always offloaded before the program. In map offloading stage, virtio_net sends control commands to qemu to create a map and return a map fd which is valid on host. This fd is stored in driver specific map structure. A list of such maps is maintained. - Currently BPF_MAP_TYPE_ARRAY and BPF_MAP_TYPE_HASH are supported. Offloading a per cpu array from guest to host doesn't make sense. Program offload: - In general the verifier in the guest replaces map fds in the user submitted programs with map pointers then bpf_prog_offload_ops callbacks are called. - This set introduces new program offload callback 'setup()' which verifier calls before replacing map fds with map pointers. This way virtio_net can create a copy of the program with guest map fds. It was needed because virtio_net wants to derive driver specific map data from guest map fd. Then guest map fd will be replaced with host map fd in the copy of the program, hence the copy of the program which will be submitted to the host will have valid host map fds. - Alternatively if we can move the prep() call in the verifier before map fd replacement happens, there is not need to introduce 'setup()' callback. - As per current implementation of 'setup()' callback in virtio_net, it verifies full program for allowed helper functions and performs above mentioned map fd replacement. - A list of allowed helper function is maintained and it is currently experimental, it will be updated later as per need. Using this list we can filter out most non-XDP type programs to some extent. Also we prevent the guest from collecting host specific information by not allowing some helper calls. - XDP_PROG_SETUP_HW is called after successful program verification. In this call a control buffer is prepared, program instructions are appended to the buffer and it is sent to qemu. tun === This set makes changes in tun to run XDP prog in Tx path. It will be the offloaded program from the guest. This program can be set using tun ioctl interface. There were multiple places where this program can be executed. - tun_net_xmit - tun_xdp_xmit - tun_recvmsg tun_recvmsg was chosen because it runs in process context. The other two run in bh context. Running in process context helps in setting up service chaining using XDP redirect. XDP_REDIRECT action of offloaded program isn't handled. It is because target interface's ndo_xdp_xmit is called when we redirect a packet. In offload case the target interface will be some tap interface. Any packet redirected towards it will sent back to the guest, which is not what we expect. Such redirect will need special handling in the kernel XDP_TX action of offloaded program is handled. Packet is injected into the Rx path in this case. Care is taken such that the tap's native Rx path XDP will be executed in such case. * Implementation in Qemu Qemu is modified to handle handle control commands from the guest. When program offload command is received, it loads the program in the host OS and attaches program fd to tap device. All the program and map operations are performed using libbpf APIs. * Performance numbers Single flow tests were performed. The diagram below shows the setup. xdp1 and xdp2 sample programs were modified to use BPF_MAP_TYPE_ARRAY instead of per cpu array and xdp1_user.c was modified to have hardware offload parameter. (Rx path XDP to drop (Tx path XDP. XDP_TX'ed pkts from Program offloaded tun Tx path XDP) from virtio_net) XDP_DROP ----------. XDP_DROP/XDP_TX \ | (Case 2) \ | XDP_DROP/XDP_TX pktgen ---> 10G-NIC === 10G-NIC --- bridge --- tun --- virtio-net |<------ netns ------>| | ^ |<----guest---->| v | '---- XDP_REDIRECT----' (Case 1) Case 1: Packets XDP_REDIRECT'ed towards tun. Non-offload Offload xdp1 (XDP_DROP) 2.46 Mpps 12.90 Mpps xdp2 (XDP_TX) 1.50 Mpps 7.26 Mpps Case 2: Packets are not redirected. They pass through the bridge. Non-offload Offload xdp1 (XDP_DROP) 1.03 Mpps 1.01 Mpps xdp2 (XDP_TX) 1.10 Mpps 0.99 Mpps In case 2, the offload performance is low. In this case the producer function is tun_net_xmit. It puts single packet in ptr ring and spends most of the time in waking up vhost thread. On the other hand, each time when vhost thread wakes up, it calls tun_recvmsg. Since Tx path XDP runs in tun_recvmsg, vhost doesn't see any packet. It sleeps frequently and producer function most spends more time in waking it up. vhost polling improves these numbers but in that case non-offload performance also improves and remains higher than the offload case. Performance in this case can be improved later in a separate work. Since this set makes changes in virtio_net, tun and vhost_net, it was necessary to measure the performance difference after applying this set. Performance numbers are in table below: Netperf Test Before After Difference UDP_STREAM 18byte 89.43 90.74 +1.46% UDP_STREAM 1472byte 6882 7026 +2.09% TCP_STREAM 9403 9407 +0.04% UDP_RR 13520 13478 -0.31% TCP_RR 13120 12918 -1.53% * Points for improvement (TODO) - In current implementation, qemu passes host map fd to the guest, which means guest is poking host information. It can be avoided by moving the map fd replacement task from guest to qemu. - Currently there is no way on the host side to show whether a tap interface has offloaded XDP program attached. - When sending program and map related control commands from guest to host, it will be better if we pass metadata about the program, map. For example BTF data. - In future virtio can have feature bit for offloading capability - TUNGETFEATURES should have a flag to notify about offloading capability - Submit virtio spec patch to describe XDP offloading feature - When offloading is enabled, it should be a migration blocker. - DoS: Offloaded map uses host's memory which is other than what has been allocated for the guest. Offloading many maps of large size can be one of the DoS strategy. Hence qemu should have parameter to limit how many maps guest can offload or how much memory offloaded maps use. * Other dependencies - Loading a bpf program requires CAP_SYS_ADMIN capability. We tested this set by running qemu as root OR adding CAP_SYS_ADMIN to the qemu binary. In other cases Qemu doesn't have this capability. Alexei's recent work CAP_BPF can be a solution to this problem. The CAP_BPF work is still being discussed in the mailing list. Jason Wang (9): bpf: introduce bpf_prog_offload_verifier_setup() net: core: rename netif_receive_generic_xdp() to do_generic_xdp_core() net: core: export do_xdp_generic_core() tun: set offloaded xdp program virtio-net: store xdp_prog in device virtio_net: add XDP prog offload infrastructure virtio_net: implement XDP prog offload functionality bpf: export function __bpf_map_get virtio_net: implment XDP map offload functionality Prashant Bhole (9): tuntap: check tun_msg_ctl type at necessary places vhost_net: user tap recvmsg api to access ptr ring tuntap: remove usage of ptr ring in vhost_net tun: run offloaded XDP program in Tx path tun: add a way to inject Tx path packet into Rx path tun: handle XDP_TX action of offloaded program tun: run xdp prog when tun is read from file interface virtio_net: use XDP attachment helpers virtio_net: restrict bpf helper calls from offloaded program drivers/net/tap.c | 42 ++- drivers/net/tun.c | 257 +++++++++++++-- drivers/net/virtio_net.c | 552 +++++++++++++++++++++++++++++--- drivers/vhost/net.c | 77 ++--- include/linux/bpf.h | 1 + include/linux/bpf_verifier.h | 1 + include/linux/if_tap.h | 5 - include/linux/if_tun.h | 23 +- include/linux/netdevice.h | 2 + include/uapi/linux/if_tun.h | 1 + include/uapi/linux/virtio_net.h | 50 +++ kernel/bpf/offload.c | 14 + kernel/bpf/syscall.c | 1 + kernel/bpf/verifier.c | 6 + net/core/dev.c | 8 +- 15 files changed, 901 insertions(+), 139 deletions(-) -- 2.20.1
