Re: Question about issue #1576: Migration from v8.0.0-rc2 to v7.2.0 with pcie-root-port device fails

[Juan Quintela <quintela@redhat.com>]

Fiona Ebner <f.ebner@proxmox.com> wrote:
> Hi,
> I'm trying to fix issue #1576 [0], but having a bit of a hard time.
>
> The issue was introduced by commit 010746ae1d ("hw/pci/aer: Implement
> PCI_ERR_UNCOR_MASK register") and the migration error is:
>> qemu-system-x86_64: get_pci_config_device: Bad config data: i=0x10a read: 40 device: 0 cmask: ff wmask: 0 w1cmask:0
>
> Since there is no dedicated subsection for the new register, but it's
> just part of the existing buffer sent along with the device, the
> approach described in docs/devel/migration.rst, "Connecting subsections
> to properties" doesn't seem to work here.
>
> AFAIU, it would be necessary to unset the new register in the buffer
> before sending for older machine types (before the patch, its value was
> 0). Then older QEMU versions should accept the migration stream again.
> Migration between >8.0 and >= 8.0 when using an older machine type would
> be softly broken, because the register value is reset to 0, but not be a
> hard error, because it's the same situation as forward migration from
> 7.2 (the error condition in get_pci_config_device() doesn't trigger,
> because versions with commit 010746ae1d set the wmask for the new
> register during device initialization).
>
> Is there a good way to unset the register conditionally based on machine
> type during save? Is there any fix for a similar issue in the past I can
> look at?
>
> Or will this be a WONTFIX, because it's just backwards migration and
> QEMU 8.0 has already been released?
>
> [0]: https://gitlab.com/qemu-project/qemu/-/issues/1576

There is a patch on list from Leonardo:

https://lists.gnu.org/archive/html/qemu-devel/2023-05/msg00350.html

Or I am missing something?

Later, Juan.