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+ The secret is using gravestones. I'll have to write up full
  documentation for the feature but essentially riscv lets us encode
  whatever we want into page table entries, as long as they're not
  active. We use this to encode highest user address that is not a zero,
  in that all entries in the top level are either active or gravestones.
  When an active entry is removed, it is either a gravestone (if there
  are other active entries above it) or it starts a cascade of removing
  entries that have been previously removed

  Slight runtime overhead to page mapping, pretty major advantage in rpc
  calls. Feature will need to be tested more thorougly, and the init
  program is sort of a best scenario with just one top level userspace
  page table entry active at a time, leading to incredibly fast context
  switches.

  Current implementation limits a process' max virtual memory to 248 GiB
  (in Sv39), but I don't think the missing 8 GiB is that big of a deal.

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