[PowerPC] Fix VSX context handling

[bdragon]

Summary

In rS356767, memcpy/memmove/bcopy optimizations were added to libc to improve performance.

This exposed an existing kernel issue in VSX handling. The PSL_VSX flag was not being excluded from the psl_userstatic set, which meant that any thread that used these and then called swapcontext(3) would get an EINVAL error.

While here, add some comments to try and explain psl_userstatic / psl_userset / psl_userset32 better, as well as add a better comment for the r344123 fix.

Test Plan

This issue can be seen in the libthr tests kill_test:simple, sigmask_test:respected_while_running, and swapcontext_test:swapcontext1, as well as in rust.

[bdragon]

ah, misread IRC, it was luporl who jhibbits was asking regarding debugging rust.

[luporl]

Nice, thanks for debugging this issue and documenting psl_userstatic / psl_userset.

sys/powerpc/aim/aim_machdep.c

258

s/requried/required/

[luporl]

Adding Renato Riolino, as he is investigating failing PowerPC tests and this fixes some of them.

[pkubaj]

Rust has started to build and cargo didn't crash (it crashed before right at the start) so I guess it's ok to commit, but I'll try to build all the ports that failed because of this bug to be sure (rust, openjdk8, openjdk11, ghc).

[pkubaj]

Hm, no, it's not ok:

db> bt
Tracing pid 64293 tid 101836 td 0xc0000000bfb13560
0xe0000000f142c530: at vpanic+0x208
0xe0000000f142c5e0: at panic+0x40
0xe0000000f142c610: at trap+0x123c
0xe0000000f142c750: at powerpc_interrupt+0x1b8
0xe0000000f142c7e0: user VSX trap by 0x804347f1c: srr1=0x900000000200d032
            r1=0x3fffffffddffa980 cr=0x44c00082 xer=0 ctr=0x16 r2=0x804364378 frame=0xe0000000f142c810
db> show panic
panic: VSX already enabled for thread

[luporl]

sys/powerpc/powerpc/exec_machdep.c

501–502

@pkubaj, maybe this change fixes the issue you're seeing:
tf->srr1 &= ~(PSL_FP | PSL_VSX);
pcb->pcb_flags &= ~(PCB_FPU | PCB_VSX);

[bdragon]

sys/powerpc/powerpc/exec_machdep.c

501–502

I *almost* went for doing that to start with, but I figured there might be a reason for the change to be FP only.

Given the kernel crash, it looks like we should definitely be doing ~(PSL_FP | PSL_VEC | PSL_VSX)

[bdragon]

Fix typos.

Always disable VSX when disabling FP.

[jhibbits]

Nice work!

[pkubaj]

For now looks good, but rust is still building, and then I'll also test openjdk8, openjdk11 and ghc.