[Bug 1869426] Re: 5.0rc0->4.2 serial migraiton

[elmarco]

I thought backward migration wasn't supported.

Isn't it this commit?

commit 4cc017e505df7e5344e4dfe7fc17711117c5f11f
Author: Marc-André Lureau <address@hidden>
Date:   Tue Oct 22 00:32:41 2019 +0200

    serial: register vmsd with DeviceClass
    
    Migration from old to new code works, however the other way fails for
    devices that use serial_init/serial_mm_init with "base", used as
    instance_id previously.
    
    (with qdev_set_legacy_instance_id, the alias_id is only used in
    savevm.c:find_se(), and thus can only be used to match against
    "legacy" instance id values. On new code, instance_id is generated
    incrementally from 0 with calculate_new_instance_id(), based on
    "qdev-path/vmsd-name")
    
    Signed-off-by: Marc-André Lureau <address@hidden>
    Reviewed-by: xiaoqiang zhao <address@hidden>

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https://bugs.launchpad.net/bugs/1869426

Title:
  5.0rc0->4.2 serial migraiton

Status in QEMU:
  New

Bug description:
  Migrating from 5.0rc0->4.2 with pc-q35-4.2 we get an error:

  Unknown savevm section or instance 'serial' 1

  dumping the migration streams it looks like 5.0 is duplicating the
  serial migration data:

      "serial (26)": {
          "divider": "0x000c",
          "rbr": "0x00",
          "ier": "0x00",
          "iir": "0x01",
          "lcr": "0x00",
          "mcr": "0x00",
          "lsr": "0x60",
          "msr": "0xb0",
          "scr": "0x00",
          "fcr_vmstate": "0x00"
      },
      "serial (27)": {
          "state": {
              "divider": "0x000c",
              "rbr": "0x00",
              "ier": "0x00",
              "iir": "0x01",
              "lcr": "0x00",
              "mcr": "0x00",
              "lsr": "0x60",
              "msr": "0xb0",
              "scr": "0x00",
              "fcr_vmstate": "0x00"
          }
      },

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