This also removes the need to do another semi-expensive remap when
cloning processes, since the KPCRs (for kernel TLS) are no longer stored
in the user PML4.
Currently, there are some things that need to be set up by userspace
that the kernel previously did. These include telling firmware when the
I/O APIC is used, and most importantly, shutting down the system.
The former is not particularly important, but for the latter I think
that we could implement this using a "shutdown pipe". Essentially it
will be a file that triggers an event shutting down, which would be used
to notify to acpid that the kernel is requesting a shutdown.
Previously, the kernel used the regular FS segment for Thread-Local
Storage. The problem however, is that userspace code also uses FS for
TLS, meaning that the kernel would have to switch the FS segment between
user and kernel, _upon every syscall_. This is obviously suboptimal for
performance (especially with fast syscalls such as futex, nanosleep, or
yield).
I had to search LLVM for hours, just to find out that the insertion of
the memory load with FS was actually done in the linker, so I added a
flag for that.
I haven't done any proper benchmarking, but the boot process seems to
have gotten much faster!
In order words, it swaps gs both directly at the start of the syscall
handler, then swaps it back, and the at the end of the syscall handler.
I cannot tell for sure why this is necessary, but probably since some
interrupt handler will execute swapgs in the wrong order or something.
4lDO2