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

[Paolo Bonzini]

> 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.

The first migration of the t->x->t sequence does not really matter (if
anything it could introduce _more_ bugs!), so if x->t is not supported
then neither is t->x->t.

The upstream QEMU project doesn't have the manpower to test and support
backwards migration.  We try not to break it, and in this case there
was an easy fix and we suggest that Canonical backports it.  However,
in general it's not guaranteed to work.

The fix is commit e6915b5f3a874a467a9a65f7ec1d6ef8d251a51a.

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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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