virtio-mem exposes a dynamic amount of memory within RAMBlocks by
coordinating with the VM. Memory within a RAMBlock can either get
plugged and consequently used by the VM, or unplugged and consequently no
longer used by the VM. Logical unplug is realized by discarding the
physical memory backing for virtual memory ranges, similar to memory
ballooning.
However, important difference to virtio-balloon are:
a) A virtio-mem device only operates on its assigned memory region /
RAMBlock ("device memory")
b) Initially, all device memory is logically unplugged
c) Virtual machines will never accidentally reuse memory that is currently
logically unplugged. The spec defines most accesses to unplugged memory
as "undefined behavior" -- except reading unplugged memory, which is
currently expected to work, but that will change in the future.
d) The (un)plug granularity is in the range of megabytes -- "memory blocks"
e) The state (plugged/unplugged) of a memory block is always known and
properly tracked.
Whenever memory blocks within the RAMBlock get (un)plugged, changes are
communicated via the RamDiscardManager to other QEMU subsystems, most
prominently vfio which updates the DMA mapping accordingly. "Unplugging"
corresponds to "discarding" and "plugging" corresponds to "populating".
While migrating (precopy/postcopy) that state of such memory blocks cannot
change, as virtio-mem will reject any guest requests that would change
the state of blocks with "busy". We don't want to migrate such logically
unplugged memory, because it can result in an unintended memory consumption
both, on the source (when reading memory from some memory backends) and on
the destination (when writing memory). Further, migration time can be
heavily reduced when skipping logically unplugged blocks and we avoid
populating unnecessary page tables in Linux.
Right now, virtio-mem reuses the free page hinting infrastructure during
precopy to exclude all logically unplugged ("discarded") parts from the
migration stream. However, there are some scenarios that are not handled
properly and need fixing. Further, there are some ugly corner cases in
postcopy code and background snapshotting code that similarly have to
handle such special RAMBlocks.
Let's reuse the RamDiscardManager infrastructure to essentially handle
precopy, postcopy and background snapshots cleanly, which means:
a) In precopy code, fixing up the initial dirty bitmaps (in the RAMBlock
and e.g., KVM) to exclude discarded ranges.
b) In postcopy code, placing a zeropage when requested to handle a page
falling into a discarded range -- because the source will never send it.
Further, fix up the dirty bitmap when overwriting it in recovery mode.
c) In background snapshot code, never populating discarded ranges, not even
with the shared zeropage, to avoid unintended memory consumption,
especially in the future with hugetlb and shmem.
Detail: When realizing a virtio-mem devices, it will register the RAM
for migration via vmstate_register_ram(). Further, it will
set itself as the RamDiscardManager for the corresponding memory
region of the RAMBlock via memory_region_set_ram_discard_manager().
Last but not least, memory device code will actually map the
memory region into guest physical address space. So migration
code can always properly identify such RAMBlocks.
Tested with precopy/postcopy on shmem, where even reading unpopulated
memory ranges will populate actual memory and not the shared zeropage.
Tested with background snapshots on anonymous memory, because other
backends are not supported yet with upstream Linux.