[$] Memory passthrough for virtual machines

Memory management is tricky enough on it own, but virtualization adds
another twist: now there are two kernels (host and guest) managing the same
memory. This duplicated effort can be wasteful if not implemented
carefully, so it is not surprising that a lot of effort, from both hardware
and software developers, has gone into this problem. As Pasha Tatashin
pointed out during a memory-management-track session at the 2023 Linux Storage, Filesystem, Memory-Management
and BPF Summit, though, there are still ways in which these systems run
less efficiently than they could. He has put some effort into improving
that situation.