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authorShane McDonald <mcdonald.shane@gmail.com>2010-05-06 23:26:57 -0600
committerRalf Baechle <ralf@linux-mips.org>2010-05-15 21:59:53 +0100
commit95e8f634d7a3ea5af40ec3fa42c8a152fd3a0624 (patch)
tree67da92dd5fa7fc27ebd25a9b524166ef182793ae
parent3f8bf8f0fd79410fbcbf9dd9910dbc9d4882c94f (diff)
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> ---
-rw-r--r--arch/mips/include/asm/mipsregs.h9
-rw-r--r--arch/mips/math-emu/cp1emu.c15
2 files changed, 19 insertions, 5 deletions
diff --git a/arch/mips/include/asm/mipsregs.h b/arch/mips/include/asm/mipsregs.h
index 49382d5e891a..c6e3c93ce7c7 100644
--- a/arch/mips/include/asm/mipsregs.h
+++ b/arch/mips/include/asm/mipsregs.h
@@ -135,6 +135,12 @@
#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
* S the sticky/flag bit
@@ -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 */
diff --git a/arch/mips/math-emu/cp1emu.c b/arch/mips/math-emu/cp1emu.c
index 8f2f8e9d8b21..f2338d1c0b48 100644
--- a/arch/mips/math-emu/cp1emu.c
+++ b/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;

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