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[Bug 1745895] Re: Unable to migrate vhost-net to virtio-1.0-capable kern


From: Launchpad Bug Tracker
Subject: [Bug 1745895] Re: Unable to migrate vhost-net to virtio-1.0-capable kernel
Date: Sat, 23 Jan 2021 04:18:32 -0000

[Expired for QEMU because there has been no activity for 60 days.]

** Changed in: qemu
       Status: Incomplete => Expired

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https://bugs.launchpad.net/bugs/1745895

Title:
  Unable to migrate vhost-net to virtio-1.0-capable kernel

Status in QEMU:
  Expired

Bug description:
  I am running QEMU 2.11 (from upstream source, not Red Hat package) on
  stock RHEL 6 and RHEL 7 kernels. Only the RHEL 7 kernel supports
  VIRTIO_F_VERSION_1 in its vhost-net driver.

  When migrating a guest using vhost-net from the RHEL 6 host to RHEL 7,
  the PCI config is rejected by QEMU on the target machine.

  A simple test case:

  1. On the RHEL 7 host, prepare for an incoming migration:

    rhel7# qemu-system-x86_64 -S -accel kvm -nographic -monitor stdio
  -nodefaults -netdev tap,id=net0,vhost=on,script=no,downscript=no
  -device virtio-net-pci,netdev=net0,mac=54:52:00:ff:ff:ff -incoming
  tcp:0.0.0.0:12345

  2. On the RHEL 6 host, start a guest and migrate it to the RHEL 7
  host:

    rhel6# qemu-system-x86_64 -S -accel kvm -nographic -monitor stdio 
-nodefaults -netdev tap,id=net0,vhost=on,script=no,downscript=no -device 
virtio-net-pci,netdev=net0,mac=54:52:00:ff:ff:ff
  QEMU 2.11.0 monitor - type 'help' for more information
    (qemu) migrate tcp:rhel7:12345

  The RHEL 7 QEMU errors out:

    qemu-system-x86_64: get_pci_config_device: Bad config data: i=0x20 read: 0 
device: c cmask: ff wmask: 0 w1cmask:0
    qemu-system-x86_64: Failed to load PCIDevice:config
    qemu-system-x86_64: Failed to load virtio-net:virtio
    qemu-system-x86_64: error while loading state for instance 0x0 of device 
'0000:00:02.0/virtio-net'
    qemu-system-x86_64: load of migration failed: Invalid argument

  If I start the source QEMU with vhost=off, or the target QEMU with
  disable-modern=true, the migration is successful.

  My hunch here is that the target QEMU prepares the PCI device to
  support VIRTIO_F_VERSION_1, as that's available in the kernel there,
  but then fails to (or does not know to) disable this during the
  migration.

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