Bug 1467797 - [virtio-win][netkvm] guest mtu in device manager is not same as host_mtu
[virtio-win][netkvm] guest mtu in device manager is not same as host_mtu
Product: Red Hat Enterprise Linux 7
Classification: Red Hat
Component: virtio-win (Show other bugs)
Unspecified Unspecified
medium Severity medium
: rc
: ---
Assigned To: Yan Vugenfirer
Virtualization Bugs
Depends On:
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Reported: 2017-07-05 04:02 EDT by lijin
Modified: 2017-07-06 08:10 EDT (History)
1 user (show)

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Last Closed: 2017-07-06 08:10:08 EDT
Type: Bug
Regression: ---
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RHEL 7.3 requirements from Atomic Host:
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Attachments (Terms of Use)
screenshot of mtu in guest (69.65 KB, image/png)
2017-07-05 04:02 EDT, lijin
no flags Details

  None (edit)
Description lijin 2017-07-05 04:02:22 EDT
Created attachment 1294485 [details]
screenshot of mtu in guest

Description of problem:

Version-Release number of selected component (if applicable):

How reproducible:

Steps to Reproduce:
1.boot guest with virtio-net-pci and set host_mtu=4000:
/usr/libexec/qemu-kvm \
  -M pc \
  -cpu host \
  -enable-kvm \
  -m 2G \
  -smp 4,cores=4 \
  -nodefconfig \
  -rtc base=localtime,driftfix=slew \
  -drive file=win2008-64-iso.qcow2,if=none,serial=virtioblk1,format=qcow2,cache=none,werror=stop,rerror=stop,id=drive-virtio-disk0,aio=native \
  -device ide-drive,drive=drive-virtio-disk0,id=virtio-disk0 \
  -device piix3-usb-uhci,id=usb \
  -device usb-tablet,id=tablet0 \
  -k en-us \
  -qmp tcp:0:4444,server,nowait \
  -boot menu=on \
  -monitor stdio \
  -cdrom en_windows_server_2008_datacenter_enterprise_standard_sp2_x64_dvd_342336.iso \
  -fda virtio-win-prewhql-0.1-140_amd64.vfd \
  -drive file=virtio-win-prewhql-0.1-140.iso,if=none,media=cdrom,id=drive-ide0-1-0,readonly=on,format=raw -device ide-drive,bus=ide.0,unit=0,drive=drive-ide0-1-0,id=ide0-1-0 \
  -vnc -vga std \
  -netdev tap,id=hostnet1,script=/etc/qemu-ifup,vhost=on -device virtio-net-pci,host_mtu=4000,netdev=hostnet1,mac=4e:63:28:bc:b1:01,id=net1

2.check guest mtu in cmd:
# netsh interface ipv4 show subinterfaces

3.check guest mtu in device manager:
device manger -> Red Hat VirtIO Ethernet Adapter -> advanced -> Init.MTUsize

Actual results:
after step2,guest mtu is 4000 as expected.
MTU  MediaSenseState   Bytes In  Bytes Out  Interface
------  ---------------  ---------  ---------  -------------
4294967295                1          0          0  Loopback Pseudo-Interface 1
  4000                1      18556      16682  Local Area Connection

after step3,guest mtu in device manager is 1500,not 4000

Expected results:
after step3,guest mtu in device manager should be same as host_mtu

Additional info:
Comment 1 Yan Vugenfirer 2017-07-06 08:10:08 EDT
This is by design.

We cannot fix or change dynamically  the MTU setting in Windows.

According to new addition in virtio spec we will read host configuration and negotiate MTU using guest settings.

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