Use maxmem=0 for disabling dynamic memory balance, instead of cryptic
service.meminfo-writer feature. Under the hood, meminfo-writer service
is also set based on maxmem property (directly in qubesdb, not
vm.features dict).
Having this as a property (not "feature"), allow to have sensible
handling of default value. Specifically, disable it automatically if
otherwise it would crash a VM. This is the case for:
- domain with PCI devices (PoD is not supported by Xen then)
- domain without balloon driver and/or meminfo-writer service
The check for the latter is heuristic (assume presence of 'qrexec' also
can indicate balloon driver support), but it is true for currently
supported systems.
This also allows more reliable control of libvirt config: do not set
memory != maxmem, unless qmemman is enabled.
memory != maxmem only makes sense if qmemman for given domain is
enabled. Besides wasting some domain resources for extra page tables
etc, for HVM domains this is harmful, because maxmem-memory difference
is made of Popupate-on-Demand pool, which - when depleted - will kill
the domain. This means domain without balloon driver will die as soon
as will try to use more than initial memory - but without balloon driver
it sees maxmem memory and doesn't know about the lower limit.
FixesQubesOS/qubes-issues#4135
4dc8631010