Here's a nasty little bug I encountered while debugging some related
problems in GDB.
Compile and run the attached program; I'm not sure if it will demonstrate
the problem on anything with hardware FPU, but at least it works on an SB-1
(using a 32-bit kernel). The program itself runs fine. Debug it with GDB,
and set a breakpoint on the ctc1 instruction. Before it executes, print out
$fsr; it will probably be 0. After trying to copy 0xf0102 into FSR, print
$fsr again; it will be 0x102. The program will still complete OK.
Now try again. After the ctc1 instruction, tell gdb "set $fsr = 0xf0102".
Then continue; the kernel locks up before the program is done.
The extra bits are two bits in the cause field, and two bits in the
reserved-write-as-zero field. I'm not sure whether setting the reserved
bits is to blame, or whether setting the cause bits raises a floating point
exception in the kernel during context switching. In any case, it looks
like we ought to be masking out some bits before saving the fcr31 value in
Description: Text Data