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Bug 1947824 - Libvirt shouldn't set the MTU of an unmanaged tap/macvtap device, it should just pass the mtu to the guest
Summary: Libvirt shouldn't set the MTU of an unmanaged tap/macvtap device, it should j...
Keywords:
Status: CLOSED CURRENTRELEASE
Alias: None
Product: Red Hat Enterprise Linux 9
Classification: Red Hat
Component: libvirt
Version: 9.0
Hardware: Unspecified
OS: Unspecified
unspecified
unspecified
Target Milestone: beta
: ---
Assignee: Laine Stump
QA Contact: Yanqiu Zhang
URL:
Whiteboard:
Depends On: 1905929
Blocks: 1904132 1920437 1924681
TreeView+ depends on / blocked
 
Reported: 2021-04-09 10:32 UTC by Yanqiu Zhang
Modified: 2021-12-07 22:02 UTC (History)
20 users (show)

Fixed In Version: libvirt-7.3.0-1.el9
Doc Type: If docs needed, set a value
Doc Text:
Clone Of: 1905929
Environment:
Last Closed: 2021-12-07 21:57:54 UTC
Type: Bug
Target Upstream Version: 7.1.0
Embargoed:
pm-rhel: mirror+

Attachments (Terms of Use)

Comment 1 Peter Krempa 2021-04-09 10:53:56 UTC 
Fixed upstream by:

commit 3bb87556b8ab010e5b808ac6775af7c10ea3d05d
Author: Laine Stump <laine>
Date:   Tue Jan 12 14:10:05 2021 -0500

    qemu: don't set interface MTU when managed='no'

v7.0.0-10-g3bb87556b8
Comment 6 Yanqiu Zhang 2021-06-07 10:21:34 UTC 
Verified on:
ibvirt-daemon-7.3.0-1.el9.x86_64
qemu-kvm-6.0.0-4.el9.x86_64

Steps:
1.set the mtu of the interface to be smaller than the the pre-created tap device
mtu of the precreated device: 2000
mtu set in the interface: 1400

# ip tuntap add mode tap name mytap0
# ip l set dev mytap0 mtu 2000
# ip l show mytap0
53: mytap0: <BROADCAST,MULTICAST> mtu 2000 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ether 46:fa:12:4f:33:be brd ff:ff:ff:ff:ff:ff

# virsh start avocado-vt-vm1 
Domain 'avocado-vt-vm1' started

# virsh dumpxml avocado-vt-vm1 |grep /inter -B8
    </controller>
    <interface type='ethernet'>
      <mac address='52:54:00:93:0f:bc'/>
      <target dev='mytap0' managed='no'/>
      <model type='virtio'/>
      <mtu size='1400'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
    </interface>

# ip l show mytap0 
53: mytap0: <BROADCAST,MULTICAST> mtu 2000 qdisc noqueue state DOWN mode DEFAULT group default qlen 1000
    link/ether 46:fa:12:4f:33:be brd ff:ff:ff:ff:ff:ff

# virsh console avocado-vt-vm1 
Connected to domain 'avocado-vt-vm1'
Escape character is ^] (Ctrl + ])

nsstest login: root
Password: 
Last login: Mon May 10 18:43:55 from 192.168.122.1
# ip l show enp1s0
2: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1400 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether 52:54:00:93:0f:bc brd ff:ff:ff:ff:ff:ff

2.set the mtu of the interface to be larger than the the pre-created tap device
mtu of the precreated device: 2000
mtu set in the interface: 3000

 # virsh edit avocado-vt-vm1 
   <interface type='ethernet'>
      <mac address='52:54:00:93:0f:bc'/>
      <target dev='mytap0' managed='no'/>
      <model type='virtio'/>
      <mtu size='3000'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
    </interface>

# virsh start avocado-vt-vm1 
Domain 'avocado-vt-vm1' started

# ip l show mytap0 
53: mytap0: <BROADCAST,MULTICAST> mtu 2000 qdisc noqueue state DOWN mode DEFAULT group default qlen 1000
    link/ether 46:fa:12:4f:33:be brd ff:ff:ff:ff:ff:ff

# virsh console avocado-vt-vm1 
Connected to domain 'avocado-vt-vm1'
Escape character is ^] (Ctrl + ])

nsstest login: root
Password: 
Last login: Mon Jun  7 18:01:28 on ttyS0
[root@nsstest ~]# ip l show enp1s0
2: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 3000 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether 52:54:00:93:0f:bc brd ff:ff:ff:ff:ff:ff
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