本文章使用Ubuntu focal 20.04 LTS作为宿主机qemu版本为5.2.0，以x86_64构架openwrt为例。
直接启动固件
因为宿主机也是x86_64构架且支持硬件虚拟化，所以这里启动kvm加速，openwrt-x86-64-generic-squashfs-combined.img是编译好的固件。
qemu-system-x86_64 --enable-kvm openwrt-x86-64-generic-squashfs-combined.img
可以看见openwrt成功运行并打印log。
Booting `OpenWrt' Booting `OpenWrt
'
[ 0.000000] Linux version 5.4.52 (fw867@koolshare.cn) (gcc version 8.4.0 (OpenWrt GCC 8.4.0 r13620-533ee970da)) #0 SMP Thu Jul 23 12:16:45 2020
[ 0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz root=PARTUUID=09cf104a-02 rootwait console=tty0 console=ttyS0,115200n8 noinitrd
[ 0.000000] x86/fpu: x87 FPU will use FXSAVE
[ 0.000000] BIOS-provided physical RAM map:
[ 0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
[ 0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
[ 0.000000] BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved
[ 0.000000] BIOS-e820: [mem 0x0000000000100000-0x0000000007fdffff] usable
[ 0.000000] BIOS-e820: [mem 0x0000000007fe0000-0x0000000007ffffff] reserved
[ 0.000000] BIOS-e820: [mem 0x00000000feffc000-0x00000000feffffff] reserved
[ 0.000000] BIOS-e820: [mem 0x00000000fffc0000-0x00000000ffffffff] reserved
[ 0.000000] NX (Execute Disable) protection: active
[ 0.000000] SMBIOS 2.8 present.
[ 0.000000] DMI: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[ 0.000000] Hypervisor detected: KVM
[ 0.000000] kvm-clock: Using msrs 4b564d01 and 4b564d00
[ 0.000000] kvm-clock: cpu 0, msr 21ea001, primary cpu clock
[ 0.000000] kvm-clock: using sched offset of 1486715385 cycles
[ 0.000003] clocksource: kvm-clock: mask: 0xffffffffffffffff max_cycles: 0x1cd42e4dffb, max_idle_ns: 881590591483 ns
[ 0.000006] tsc: Detected 3693.044 MHz processor
[ 0.001576] last_pfn = 0x7fe0 max_arch_pfn = 0x400000000
[ 0.001634] x86/PAT: Configuration [0-7]: WB WC UC- UC WB WP UC- WT
[ 0.003922] found SMP MP-table at [mem 0x000f5ab0-0x000f5abf]
[ 0.004170] ACPI: Early table checksum verification disabled
[ 0.004174] ACPI: RSDP 0x00000000000F58D0 000014 (v00 BOCHS )
[ 0.004189] ACPI: RSDT 0x0000000007FE1550 000034 (v01 BOCHS BXPCRSDT 00000001 BXPC 00000001)
[ 0.004193] ACPI: FACP 0x0000000007FE1404 000074 (v01 BOCHS BXPCFACP 00000001 BXPC 00000001)
[ 0.004199] ACPI: DSDT 0x0000000007FE0040 0013C4 (v01 BOCHS BXPCDSDT 00000001 BXPC 00000001)
[ 0.004201] ACPI: FACS 0x0000000007FE0000 000040
[ 0.004204] ACPI: APIC 0x0000000007FE1478 000078 (v01 BOCHS BXPCAPIC 00000001 BXPC 00000001)
[ 0.004206] ACPI: HPET 0x0000000007FE14F0 000038 (v01 BOCHS BXPCHPET 00000001 BXPC 00000001)
[ 0.004209] ACPI: WAET 0x0000000007FE1528 000028 (v01 BOCHS BXPCWAET 00000001 BXPC 00000001)
[ 0.004420] Zone ranges:
[ 0.004423] DMA [mem 0x0000000000001000-0x0000000000ffffff]
[ 0.004425] DMA32 [mem 0x0000000001000000-0x0000000007fdffff]
[ 0.004426] Normal empty
[ 0.004427] Movable zone start for each node
[ 0.004428] Early memory node ranges
[ 0.004429] node 0: [mem 0x0000000000001000-0x000000000009efff]
[ 0.004431] node 0: [mem 0x0000000000100000-0x0000000007fdffff]
[ 0.004434] Zeroed struct page in unavailable ranges: 130 pages
[ 0.004435] Initmem setup node 0 [mem 0x0000000000001000-0x0000000007fdffff]
[ 0.005180] ACPI: PM-Timer IO Port: 0x608
[ 0.005193] ACPI: LAPIC_NMI (acpi_id[0xff] dfl dfl lint[0x1])
[ 0.005229] IOAPIC[0]: apic_id 0, version 17, address 0xfec00000, GSI 0-23
[ 0.005232] ACPI: INT_SRC_OVR (bus 0 bus_irq 0 global_irq 2 dfl dfl)
[ 0.005234] ACPI: INT_SRC_OVR (bus 0 bus_irq 5 global_irq 5 high level)
[ 0.005235] ACPI: INT_SRC_OVR (bus 0 bus_irq 9 global_irq 9 high level)
[ 0.005236] ACPI: INT_SRC_OVR (bus 0 bus_irq 10 global_irq 10 high level)
[ 0.005237] ACPI: INT_SRC_OVR (bus 0 bus_irq 11 global_irq 11 high level)
.........
[ 8.104817] NET: Registered protocol family 24
[ 8.106891] PPTP driver version 0.8.5
[ 8.109690] usbcore: registered new interface driver r8152
[ 8.114840] sky2: driver version 1.30
[ 8.116913] tulip: Linux Tulip driver version 1.1.15-NAPI (Feb 27, 2007)
[ 8.118984] usbcore: registered new interface driver ums-alauda
[ 8.121039] usbcore: registered new interface driver ums-cypress
[ 8.122997] usbcore: registered new interface driver ums-datafab
[ 8.124969] usbcore: registered new interface driver ums-freecom
[ 8.126806] usbcore: registered new interface driver ums-isd200
[ 8.128691] usbcore: registered new interface driver ums-jumpshot
[ 8.130555] usbcore: registered new interface driver ums-karma
[ 8.132395] usbcore: registered new interface driver ums-sddr09
[ 8.134289] usbcore: registered new interface driver ums-sddr55
[ 8.136075] usbcore: registered new interface driver ums-usbat
[ 8.141246] usbcore: registered new interface driver usblp
[ 8.146016] wireguard: WireGuard 1.0.20200712 loaded. See www.wireguard.com for information.
[ 8.147770] wireguard: Copyright (C) 2015-2019 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
[ 8.212805] xt_ndpi v1.2 ndpi 3.3.0 IPv6=YES debug_message=no
[ 8.212805] BT: hash_size 0k, hash_expiation 0 sec, log_size 128kb
[ 8.212805] sizeof hash_ip4p_node=44 id_struct=256
[ 8.212805] flow_struct=2264 packet_struct=1416
[ 8.212805] flow_tcp_struct=96 flow_udp_struct=36 int_one_line_struct=16
[ 8.212805] ndpi_ip_addr_t=16 ndpi_protocol=4 nf_ct_ext_ndpi=176
[ 8.212805] flow_info=112 spinlock_t=4 NF_EXT_ID 7
[ 8.222766] xt_ndpi: MAX_PROTOCOLS 320 LAST_PROTOCOL 253
[ 8.224190] xt_ndpi: flow acctounting OFF
[ 8.258639] xt_time: kernel timezone is -0000
[ 8.261218] usbcore: registered new interface driver asix
[ 8.263491] usbcore: registered new interface driver ax88179_178a
[ 8.295036] l2tp_ppp: PPPoL2TP kernel driver, V2.0
[ 8.297720] PCLMULQDQ-NI instructions are not detected.
[ 8.299826] kmodloader: 3 modules could not be probed
[ 8.301340] kmodloader: - crc32-pclmul - 0
[ 8.302581] kmodloader: - crct10dif-pclmul - 0
[ 8.303918] kmodloader: - ghash-clmulni-intel - 0
[ 9.462693] ip_local_port_range: prefer different parity for start/end values.
[ 10.091483] xt_FULLCONENAT: RFC3489 Full Cone NAT module
[ 10.091483] xt_FULLCONENAT: Copyright (C) 2018 Chion Tang <tech@chionlab.moe>
[ 10.671267] 8021q: adding VLAN 0 to HW filter on device eth0
[ 10.704309] br-lan: port 1(eth0) entered blocking state
[ 10.705797] br-lan: port 1(eth0) entered disabled state
[ 10.759109] e1000: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: RX
[ 10.805954] br-lan: port 1(eth0) entered blocking state
[ 10.807458] br-lan: port 1(eth0) entered forwarding state
[ 10.847259] IPv6: ADDRCONF(NETDEV_CHANGE): br-lan: link becomes ready
BusyBox v1.31.1 () built-in shell (ash)
_______ ________ __
| |.-----.-----.-----.| | | |.----.| |_
| - || _ | -__| || | | || _|| _|
|_______|| __|_____|__|__||________||__| |____|
|__| W I R E L E S S F R E E D O M
-----------------------------------------------------
Openwrt Koolshare mod V2.36, r14941-67f6fa0a30 by fw867
-----------------------------------------------------
root@Openwrt:/#
网桥配置
openwrt经常需要调试和网络相关的功能，所以需要虚拟机和物理机之间网络可以互通而qemu默认的网络配置虚拟机和物理机之间无法之间互通，这时应该配置网桥来使虚拟机和物理机的网络能够互相联通。
注意：不同发行版的Linux网桥配置方法有可能不一样，这里仅介绍Ubuntu focal 20.04 LTS。
Ubuntu focal 20.04 LTS使用netplan来管理网络，编辑/etc/netplan/xxx.yaml即可配置网络，这里贴出本人物理机上的网桥配置：
# Let NetworkManager manage all devices on this system
network:
ethernets:
eno1:
dhcp4: true
bridges:
br0:
interfaces: [eno1]
dhcp4: false
addresses: [192.168.1.192/24]
gateway4: 192.168.1.2
nameservers:
addresses: [192.168.1.2,8.8.8.8]
version: 2
eno1是物理机上的物理网卡，br0为创建的网桥，配置完成后使用sudo netplan apply命令可以应用配置，以下是应用后我的物理机ifconfig输出的log:
br0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.1.192 netmask 255.255.255.0 broadcast 192.168.1.255
inet6 fdd5:36d5:eb21:0:b62e:99ff:feea:18d2 prefixlen 64 scopeid 0x0<global>
inet6 fe80::b62e:99ff:feea:18d2 prefixlen 64 scopeid 0x20<link>
ether b4:2e:99:ea:18:d2 txqueuelen 1000 (Ethernet)
RX packets 1493623 bytes 2479701090 (2.4 GB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 1013030 bytes 91124015 (91.1 MB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
eno1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
ether b4:2e:99:ea:18:d2 txqueuelen 1000 (Ethernet)
RX packets 1925874 bytes 2579666834 (2.5 GB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 1122528 bytes 107627799 (107.6 MB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1000 (Local Loopback)
RX packets 30575 bytes 3431746 (3.4 MB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 30575 bytes 3431746 (3.4 MB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
可以看见有个br0网桥出现。
在qemu上使用网桥
sudo qemu-system-x86_64 --enable-kvm openwrt-x86-64-generic-squashfs-combined.img -device e1000,netdev=brdev0 -netdev bridge,id=brdev0,br=br0
这条命令模拟了一个e1000网卡桥接到物理机的br0，br0相当于一个交换机。
在openwrt的/etc/config/network文件中配置好ip然后重启虚拟机即可实现物理机与虚拟机的网络互通。