[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]

I backported this and tried my tests again, but this alone isn't sufficient to 
get the T->X->T working (which is effectively 2.0->2.5->2.0).
Wily (2.4) is already out of service, so setting this to won't fix.

Thanks for your guidance, but that now properly known I'll set the
Xenial task to won't fix for now.

** Changed in: qemu (Ubuntu Xenial)
       Status: Triaged => Won't Fix

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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:
  Won't Fix

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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