Re: [Qemu-devel] [PATCH v4 2/7] fw-cfg: turn FW_CFG_FILE_SLOTS into a device property

[Gerd Hoffmann]

On Do, 2016-12-01 at 18:06 +0100, Laszlo Ersek wrote:
> We'd like to raise the value of FW_CFG_FILE_SLOTS. Doing it naively could
> lead to problems with backward migration: a more recent QEMU (running an
> older machine type) would allow the guest, in fw_cfg_select(), to select a
> high key value that is unavailable in the same machine type implemented by
> the older (target) QEMU.

I don't think we need this.  fw_cfg changes are guest-visible so they
must not happen for a given machine type.  So if current machine types
don't hit the limit that should continue to be the case even if we
simply raise FW_CFG_FILE_SLOTS.

But we have to take care that new files show up on new machine types
only.

cheers,
  Gerd