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+ Should write this down somewhere but the idea is that when a process
gets killed, it frees all the memory it can, making all threads within
that process orphans. Orphaned threads are assigned to the init
process, which will generally call exit() on each one. Zombie threads
are threads that own some bit of shared data, and whose reference
count is above zero. They may not be swapped to or called, even though
they take up space in thread map and reserve their thread ID.
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+ Gives a very slight improvement to RPC speeds, but mostly cleans up
code a little bit.
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+ Fairly considerable speedup, as we don't have to look up the rpc pte
every time separately, instead cacheing them. Adds an architecture
specific limitation to total rpc stack size, though.
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+ Found it in newlib. This allows the compiler to more effectively
optimize away unnecessary register operations. Applied to both kernel
and init.c for a slight speed increase.
I really should move init.c to some other repository though, the
binary files are starting to get annoying
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+ Make 8250 serial driver more generic
+ Still TODO: write a tutorial on how to boot on the visionfive2
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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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+ 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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