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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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+ Not strictly done yet, but we can swap between two processes :D
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+ Slightly unsure if it should be a syscall, or if assign would be
enough. Also, which thread should run in a fork/spawn? For now, old
thread, though this might increase latency. Or add swap flag to these
calls?
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+ In total, a syscall is built up of 6 values, with the first being the
syscall number. Symmetrically, the first value is now a status and the
following five values return "values". This allows us to cram in more
info into the ipc_* functions. The performance difference is
absolutely minimal, at least from my testing in qemu.
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+ Essentially, separate root process and remote procedure call handling.
I really should write down some documentation so I don't forget, but
essentially: All threads share a common process virtual memory, and
when a thread wants to make an rpc, it switches over to its own rpc
vmem space that is linked to the remote process.
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+ Next step, start documenting contents of each file.
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+ Closer to what it's for rather than what it is.
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+ next up would probably be to start working on process/thread
relationships, largely as documented. Still not entirely sure about
how I want to handle fork, but I suppose I might figure it out at some
point.
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+ No clue why I had that stuff in common/main.c
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