Turns out the problem all along was that the ActivePageTable was never
dropped in usermode_bootstrap. So as soon as any other hardware thread
tried to do page table business, it deadlocked!
This does the same as the previous MR, but fixes the issue where the
parent process got the mapping (and at the wrong address) and not the
child process.
Multi-core is slightly broken when using the latest version of spin
(0.9.2). I believe this is because Once used to do SeqCst loads/stores
everywhere, which might have made any possible data race much harder to
come by.
Note that this is very preliminary, and I merely got my already freezing
kernel branch not to triple fault, but I would probably apply this patch
to upstream.
What is changed here, is that rather than relying on recursive mapping
for accessing page table frames, it now uses linear translation
(virt=phys+KERNEL_OFFSET). The only problem is that the paging code now
makes assumptions that the entire physical address space remains mapped,
which is not necessarily the case on x86_64 architecturally, even though
systems with RAM more than a PML4 are very rare. We'd probably lazily
(but linearly) map physical address space using huge pages.
Previously context::switch used compare_and_swap for acquiring the
global context switch lock, but given its deprecation in more recent
Rust versions, it has been replaced with compare_exchange_weak (which
can be further optimized on some architectures).
It also replaces panic!() with abort() in switch_finish_hook, because
unwinding from assembly is not that fun.
The reason for these types of rewrites, is that more recent Rust
compilers have started to deprecate naked functions that consist of more
than only a single asm block, as they can trigger all sorts of UB.
Previously there was a triple fault, due to a combination of reasons
(e.g. rsp and rbp being ordered in the struct and in the assembly).
Now, the locks will be held __all the way until the new context__ has
been switched to, which completely eliminates any possibility that the
"pcid fault" originates here.
While I am unsure whether this will work, this could also be an
opportunity to be able to remove CONTEXT_SWITCH_LOCK fully.
This is due to a warning in more recent compilers, which forbid anything
but a single inline assembly block, in naked functions. It does
unfortunately triple fault right now, but I hope I may be able to fix it
soon.
4lDO2