Re: [PATCH v7 2/5] softmmu/vl: Let -fw_cfg option take a 'gen_id' argument

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

> > I can explain the rationale for that change, but I'm not sure of the
> > answer to your question.  That changes makes sure that the fw_cfg data
> > remains exactly the same even on newer versions of qemu if the machine
> > is set the same.  This way you can do migrations to newer qemu versions
> > and anything using fw_cfg won't get confused because the data changes.
> > 
> > The reason that change was so complex was preserving the order for
> > migrating from older versions.
> > 
> > This is only about migration.  I'm not sure what gen_id is, but if it's
> > migrated, it better be future proof.
> 
> Whenever introducing a new fw_cfg file (*any* new named file), how do we
> decide whether we need fw_cfg_set_order_override()?

The whole point of the sorting is to make sure the fw_cfg directory
listing entry (FW_CFG_FILE_DIR) is stable and doesn't change underneath
the guest when it gets live-migrated.

That sorting was added in qemu 2.6, to make sure things don't chance by
accident in case the initialization order changes.  Now you've got a
problem when you migrate from qemu 2.5 (+older) to qemu 2.6 (+newer),
because 2.5 has the entries in initialization order and 2.6 has the
entries in alphabetical order.  To deal with that machine types for 2.5
& older keep the old sort order.  This is the reason why
legacy_fw_cfg_order exists.

For new features and files you can completely ignore the whole legacy
sorting mess.  cross-version live migration works only for features
supported by both qemu versions, therefore the legacy sorting is only
relevant for features & files already supported by qemu 2.5.

HTH,
  Gerd