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+ Now 10M alloc/frees succeed, which totals more memory than the virtual
machine has, so no too obvious leaks are occuring. For future
debugging speed, changed 10M to 1M.
+ Also quick fix to rpcs, stacks are now assigned. Not entirely sure why
they worked before this, but good that I found it.
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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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+ Eliminated need for save/load_regs, now the register save area is
behind a pointer which _slightly_ increases overall syscall delay, but
speeds up ipc requests by a fair bit.
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+ It really only works with Sv39, I suppose similar structures should be
added for Sv4} etc. if I ever get around to it.
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+ Both easier to look at and now the userspace doesn't play as big of a
role in the 'benchmarking' with optimizations turned on.
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+ Not strictly done yet, but we can swap between two processes :D
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+ Not actually working (at least fully), need to continue debugging when
I have time.
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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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+ Note that it is NOT intended for end user usage, just for debugging.
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+ Prepare for possibly testing rv32
+ Also remove newline from debugging output
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+ May warn about PHDR not being in LOAD, there is a flag to silence it
(--no-warn-rwx-segment) but it doesn't seem to exist in
riscv64-unknown-elf for some reason.
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+ Doesn't seem to be specified anywhere, but I'll stick with 2MB for
now, working on the assumption that the FW is at the start of the
physical RAM.
Possible TODOs: generate apos.its automatically from init.elf,
allocate root_branch statically?
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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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