Migrate meminfo-writer=False service setting to maxmem=0 as a method to
disable dynamic memory management. Remove the service from vm.features
dict in the process.
Additionally, translate any attempt to set the service.meminfo-writer
feature to either setting maxmem=0 or resetting it to the default (which
is memory balancing enabled if supported by given domain). This is to at
least partially not break existing tools using service.meminfo-writer as
a way to control dynamic memory management. This code does _not_ support
reading service.meminfo-writer feature state to get the current state of
dynamic memory management, as it would require synchronizing with all
the factors affecting its value. One of main reasons for migrating to
maxmem=0 approach is to avoid the need of such synchronization.
QubesOS/qubes-issues#4480
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
Support 'supported-service.*' features requests coming from VMs. Set
such features directly (allow only value '1') and remove any not
reported in given call. This way uninstalling package providing given
service will automatically remove related 'supported-service...'
feature.
FixesQubesOS/qubes-issues#4402
Rename events:
- domain-feature-set -> domain-feature-set:feature
- domain-feature-delete -> domain-feature-delete:feature
- domain-tag-add -> domain-tag-add:tag
- domain-tag-delete -> domain-tag-delete:tag
Make it consistent with property-* events. It makes more sense to
include tag/feature name in event name, so handler can watch a single
tag/feature - which is the most common case. Otherwise, most handlers
would begin with `if feature == '...'` anyway, wasting time on most
events.
In cases where multiple features/tags should be handled by a single
handler, it is now possible to register a handler with wildcard, for
example `domain-feature-set:*`.
