[Qemu-devel] [Bug 1536487] Re: Unable to migrate pc-i440fx-2.4 KVM guest from QEMU 2.5.0 to QEMU 2.4.1

[ChristianEhrhardt]

Hi Serge I agree to "created on xenial -> migrating to trusty" not being 
supported.
I already tended to even say "created on xenial with the Trusty machine type -> 
migrating to trusty" is not supported as well (at least it is failing for all 
combos I tried.

But I wonder how far "anything moving backward" should go.

Especially I found that the "created on Trusty, migrated to xenial (works), but 
later migrated back to trusty (fails)" seems affected by it as well.
I'd have thought that this would be supported. What is you opinion on this more 
specific case?

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https://bugs.launchpad.net/bugs/1536487

Title:
  Unable to migrate pc-i440fx-2.4 KVM guest from QEMU 2.5.0 to QEMU
  2.4.1

Status in QEMU:
  Fix Released
Status in qemu package in Ubuntu:
  Fix Released
Status in qemu source package in Xenial:
  Triaged

Bug description:
  When migrating a pc-i440fc-2.4 KVM guest from QEMU 2.5.0 to QEMU
  2.4.1, the target QEMU errors out:

    qemu-system-x86_64: error while loading state for instance 0x0 of
  device 'fw_cfg'

  This appears to be related to the addition of a DMA interface to
  fw_cfg last October:

    http://lists.nongnu.org/archive/html/qemu-
  devel/2015-10/msg04568.html

  "info qtree" on the source QEMU shows that the DMA interface for
  fw_cfg had been enabled:

    bus: main-system-bus
      type System
      ...
      dev: fw_cfg_io, id ""
        iobase = 1296 (0x510)
        dma_iobase = 1300 (0x514)
        dma_enabled = true

  Incidentally, this guest had just undergone a migration from QEMU
  2.4.1 to QEMU 2.5.0, so it looks like DMA was enabled simply through
  the migration.

  It seems to me that the DMA interface for fw_cfg should only be
  enabled on pc-i440fx-2.5 machines or higher.

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