MIPS FPU emulator: allow Cause bits of FCSR to be writeable by ctc1

    
    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: Shane 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: Ralf Baechle <ralf@linux-mips.org>

---

arch/mips/include/asm/mipsregs.h

@@ -134,6 +134,12 @@
 #define FPU_CSR_COND6   0x40000000      /* $fcc6 */
 #define FPU_CSR_COND7   0x80000000      /* $fcc7 */
 
+/*
+ * Bits 18 - 20 of the FPU Status Register will be read as 0,
+ * and should be written as zero.
+ */
+#define FPU_CSR_RSVD	0x001c0000
+
 /*
  * X the exception cause indicator
  * E the exception enable
@@ -161,7 +167,8 @@
 #define FPU_CSR_UDF_S   0x00000008
 #define FPU_CSR_INE_S   0x00000004
 
-/* rounding mode */
+/* Bits 0 and 1 of FPU Status Register specify the rounding mode */
+#define FPU_CSR_RM	0x00000003
 #define FPU_CSR_RN      0x0     /* nearest */
 #define FPU_CSR_RZ      0x1     /* towards zero */
 #define FPU_CSR_RU      0x2     /* towards +Infinity */

arch/mips/math-emu/cp1emu.c

@@ -78,6 +78,9 @@ DEFINE_PER_CPU(struct mips_fpu_emulator_stats, fpuemustats);
 #define FPCREG_RID	0	/* $0  = revision id */
 #define FPCREG_CSR	31	/* $31 = csr */
 
+/* Determine rounding mode from the RM bits of the FCSR */
+#define modeindex(v) ((v) & FPU_CSR_RM)
+
 /* Convert Mips rounding mode (0..3) to IEEE library modes. */
 static const unsigned char ieee_rm[4] = {
 	[FPU_CSR_RN] = IEEE754_RN,
@@ -384,10 +387,14 @@ static int cop1Emulate(struct pt_regs *xcp, struct mips_fpu_struct *ctx)
 					(void *) (xcp->cp0_epc),
 					MIPSInst_RT(ir), value);
 #endif
-				value &= (FPU_CSR_FLUSH | FPU_CSR_ALL_E | FPU_CSR_ALL_S | 0x03);
-				ctx->fcr31 &= ~(FPU_CSR_FLUSH | FPU_CSR_ALL_E | FPU_CSR_ALL_S | 0x03);
-				/* convert to ieee library modes */
-				ctx->fcr31 |= (value & ~0x3) | ieee_rm[value & 0x3];
+
+				/*
+				 * Don't write reserved bits,
+				 * and convert to ieee library modes
+				 */
+				ctx->fcr31 = (value &
+						~(FPU_CSR_RSVD | FPU_CSR_RM)) |
+						ieee_rm[modeindex(value)];
 			}
 			if ((ctx->fcr31 >> 5) & ctx->fcr31 & FPU_CSR_ALL_E) {
 				return SIGFPE;