spapr: Simplify handling of host-serial and host-model values
27461d69 "ppc: add host-serial and host-model machine attributes (CVE-2019-8934)" introduced 'host-serial' and 'host-model' machine properties for spapr to explicitly control the values advertised to the guest in device tree properties with the same names. The previous behaviour on KVM was to unconditionally populate the device tree with the real host serial number and model, which leaks possibly sensitive information about the host to the guest. To maintain compatibility for old machine types, we allowed those props to be set to "passthrough" to take the value from the host as before. Or they could be set to "none" to explicitly omit the device tree items. Special casing specific values on what's otherwise a user supplied string is very ugly. So, this patch simplifies things by implementing the backwards compatibility in a different way: we have a machine class flag set for the older machines, and we only load the host values into the device tree if A) they're not set by the user and B) we have that flag set. This does mean that the "passthrough" functionality is no longer available with the current machine type. That's ok though: if a user or management layer really wants the information passed through they can read it themselves (OpenStack Nova already does something similar for x86). It also means the user can't explicitly ask for the values to be omitted on the old machine types. I think that's an acceptable trade-off: if you care enough about not leaking the host information you can either move to the new machine type, or use a dummy value for the properties. For the new machine type, this also removes an odd inconsistency between running on a POWER and non-POWER (or non-Linux) hosts: if the host information couldn't be read from where we expect (in the host's device tree as exposed by Linux), we'd fallback to omitting the guest device tree items. While we're there, improve some poorly worded comments, and the help text for the properties. Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au> Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com> Reviewed-by: NGreg Kurz <groug@kaod.org> Tested-by: NGreg Kurz <groug@kaod.org>
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