• Shane McDonald's avatar
    MIPS FPU emulator: allow Cause bits of FCSR to be writeable by ctc1 · 95e8f634
    Shane McDonald authored
        In the FPU emulator code of the MIPS, the Cause bits of the FCSR register
        are not currently writeable by the ctc1 instruction.  In odd corner cases,
        this can cause problems.  For example, a case existed where a divide-by-zero
        exception was generated by the FPU, and the signal handler attempted to
        restore the FPU registers to their state before the exception occurred.  In
        this particular setup, writing the old value to the FCSR register would
        cause another divide-by-zero exception to occur immediately.  The solution
        is to change the ctc1 instruction emulator code to allow the Cause bits of
        the FCSR register to be writeable.  This is the behaviour of the hardware
        that the code is emulating.
        This problem was found by Shane McDonald, but the credit for the fix goes
        to Kevin Kissell.  In Kevin's words:
        I submit that the bug is indeed in that ctc_op:  case of the emulator.  The
        Cause bits (17:12) are supposed to be writable by that instruction, but the
        CTC1 emulation won't let them be updated by the instruction.  I think that
        actually if you just completely removed lines 387-388 [...] things would
        work a good deal better.  At least, it would be a more accurate emulation of
        the architecturally defined FPU.  If I wanted to be really, really pedantic
        (which I sometimes do), I'd also protect the reserved bits that aren't
        necessarily writable.
    Signed-off-by: default avatarShane McDonald <mcdonald.shane@gmail.com>
        To: anemo@mba.ocn.ne.jp
        To: kevink@paralogos.com
        To: sshtylyov@mvista.com
        Patchwork: http://patchwork.linux-mips.org/patch/1205/
    Signed-off-by: default avatarRalf Baechle <ralf@linux-mips.org>