QEMU configuration¶

Important

A bug in some versions of QEMU prevents mergeable buffers to be correctly negotiated when using a vhost-user netdevice. The following patch, backported in QEMU v2.1.3, fixes the bug in QEMU v2.2.0: http://git.qemu.org/?p=qemu.git;a=commitdiff;h=d8e80ae37a7acfea416ad9abbe76b453a73d9cc0
A bug in some versions of QEMU prevents the vhost user interface to properly negotiate supported features, causing the device to fail initialization. The following patch, backported in QEMU v2.1.1, fixes the bug in QEMU v2.2.0: http://git.qemu.org/?p=qemu.git;a=commitdiff;h=b49ae9138d5cadb47fb868297fbcdac8292fb666
Start the VM with memory allocated from hugepages in shared mode with the following configuration (the vhost-user PMD must be able to map the VM’s memory to write in the virtio ring):
```
-object memory-backend-file,id=mem,size=2G,mem-path=/mnt/huge-2M,share=on \
-numa node,memdev=mem
```
A bug in QEMU < v5.2.0 when multi-queues are used breaks the connectivity from VM to VM after a VA restart. The following patch https://github.com/qemu/qemu/commit/f66337bdbfdae6a288f37dcfba3748ee0015510d fixes the problem which is part of QEMU v5.2.0.

Start a vhost-user netdevice with QEMU:
```
-chardev socket,id=vhostusernet0,path=/tmp/vhost_sock0[,server] \
-netdev type=vhost-user,id=vhostusernet0,chardev=vhostuserchr0,queues=8 \
-device virtio-net-pci,netdev=vhostusernet0,ioeventfd=on,vectors=17,mq=on
```
path
Path to the vhost-user socket.

server
Create the vhost-user socket in server mode. This option needs to be set if the Virtio Host PMD socket is configured in client mode.

queues=x
Number of queue pairs seen by the driver on the guest. This requires a patch in QEMU, provided in librte_pmd_vhost/patches/qemu-2.x.

vectors=x
Required for multiqueue; should be NQ * 2 + 1.

mq=on
Enables multiqueue.

ioeventfd=on
Should increase notification performance when the guest driver is a Linux driver
[Optional] Modify the set of negotiated features.

By default, QEMU automatically enables all options supported by both sides (checksum offloading, GSO, mergeable buffers, and so on). To modify the set of negotiated features, you can edit the following parameters:

guest_csum=on/off
Offload l4 checksum from host to guest (default: on)

guest_tso4=on/off
Forward TSO4 packets from host to guest (default: on)

guest_tso6=on/off
Forward TSO6 packets from host to guest (default: on)

csum=on/off
Offload l4 checksum from guest to host (default: on)

host_tso4=on/off
Forward TSO4 packets from guest to host (default: on)

host_tso6=on/off
Forward TSO6 packets from guest to host (default: on)

mrg_rxbuf=on/off
Support mergeable buffers in host (default: on)

Display negotiated features via the QEMU monitor:

(qemu) info qtree
bus: main-system-bus
  type System
  dev: hpet, id ""
    gpio-in "" 2
    gpio-out "" 1
    timers = 3 (0x3)
    msi = false
    hpet-intcap = 4 (0x4)
    irq 32
    mmio 00000000fed00000/0000000000000400
  dev: kvm-ioapic, id ""
    gpio-in "" 24
    gsi_base = 0 (0x0)
    irq 0
    mmio 00000000fec00000/0000000000001000
  dev: i440FX-pcihost, id ""
    pci-hole64-size = 18446744073709551615 (16 EiB)
    short_root_bus = 0 (0x0)
    irq 0
    bus: pci.0
      type PCI
      dev: virtio-net-pci, id ""
        ioeventfd = true
        vectors = 3 (0x3)
        any_layout = false
        csum = true
        guest_csum = true
        gso = false
        guest_tso4 = true
        guest_tso6 = true
        guest_ecn = false
        guest_ufo = false
        guest_announce = true
        host_tso4 = true
        host_tso6 = true
        host_ecn = false
        host_ufo = false
        mrg_rxbuf = true
        status = true
        ctrl_vq = true
        ctrl_rx = true
        ctrl_vlan = false
        ctrl_rx_extra = false
        ctrl_mac_addr = true
        ctrl_guest_offloads = false
        mq = false
        mac = "02:11:22:00:06:00"
        vlan = <null>
        netdev = "vhostusernet0"
        bootindex = -1 (0xffffffffffffffff)
        x-txtimer = 150000 (0x249f0)
        x-txburst = 256 (0x100)
        tx = ""
        indirect_desc = false
        event_idx = false
        addr = 04.0
        romfile = "efi-virtio.rom"
        rombar = 1 (0x1)
        multifunction = false
        command_serr_enable = true
        class Ethernet controller, addr 00:04.0, pci id 1af4:1000 (sub 1af4:0001)
        bar 0: i/o at 0xc100 [0xc11f]
        bar 1: mem at 0xfebd1000 [0xfebd1fff]
        bar 6: mem at 0xffffffffffffffff [0x3fffe]
        bus: virtio-bus
          type virtio-pci-bus
          dev: virtio-net-device, id ""
            mac = "02:11:22:00:06:00"
            vlan = <null>
            netdev = "vhostusernet0"
            bootindex = -1 (0xffffffffffffffff)
            x-txtimer = 150000 (0x249f0)
            x-txburst = 256 (0x100)
            tx = ""
      dev: ne2k_pci, id ""
        mac = "de:ad:de:01:02:03"
        vlan = <null>
        netdev = "user.0"
        bootindex = -1 (0xffffffffffffffff)
        addr = 03.0
        romfile = "efi-ne2k_pci.rom"
        rombar = 1 (0x1)
        multifunction = false
        command_serr_enable = true
        class Ethernet controller, addr 00:03.0, pci id 10ec:8029 (sub 1af4:1100)
        bar 0: i/o at 0xc000 [0xc0ff]
        bar 6: mem at 0xffffffffffffffff [0x3fffe]
        ...