target/arm: Make FPSCR/FPCR trapped-exception bits RAZ/WI

The {IOE, DZE, OFE, UFE, IXE, IDE} bits in the FPSCR/FPCR are for
enabling trapped IEEE floating point exceptions (where IEEE exception
conditions cause a CPU exception rather than updating the FPSR status
bits). QEMU doesn't implement this (and nor does the hardware we're
modelling), but for implementations which don't implement trapped
exception handling these control bits are supposed to be RAZ/WI.
This allows guest code to test for whether the feature is present
by trying to write to the bit and checking whether it sticks.

QEMU is incorrectly making these bits read as written. Make them
RAZ/WI as the architecture requires.

In particular this was causing problems for the NetBSD automatic
test suite.

Reported-by: Martin Husemann <martin@netbsd.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190131130700.28392-1-peter.maydell@linaro.org

target/arm/cpu.h

@@ -1418,6 +1418,12 @@ void vfp_set_fpscr(CPUARMState *env, uint32_t val);
 #define FPSR_MASK 0xf800009f
 #define FPCR_MASK 0x07ff9f00
 
+#define FPCR_IOE    (1 << 8)    /* Invalid Operation exception trap enable */
+#define FPCR_DZE    (1 << 9)    /* Divide by Zero exception trap enable */
+#define FPCR_OFE    (1 << 10)   /* Overflow exception trap enable */
+#define FPCR_UFE    (1 << 11)   /* Underflow exception trap enable */
+#define FPCR_IXE    (1 << 12)   /* Inexact exception trap enable */
+#define FPCR_IDE    (1 << 15)   /* Input Denormal exception trap enable */
 #define FPCR_FZ16   (1 << 19)   /* ARMv8.2+, FP16 flush-to-zero */
 #define FPCR_FZ     (1 << 24)   /* Flush-to-zero enable bit */
 #define FPCR_DN     (1 << 25)   /* Default NaN enable bit */

target/arm/helper.c

@@ -12637,6 +12637,12 @@ void HELPER(vfp_set_fpscr)(CPUARMState *env, uint32_t val)
         val &= ~FPCR_FZ16;
     }
 
+    /*
+     * We don't implement trapped exception handling, so the
+     * trap enable bits are all RAZ/WI (not RES0!)
+     */
+    val &= ~(FPCR_IDE | FPCR_IXE | FPCR_UFE | FPCR_OFE | FPCR_DZE | FPCR_IOE);
+
     changed = env->vfp.xregs[ARM_VFP_FPSCR];
     env->vfp.xregs[ARM_VFP_FPSCR] = (val & 0xffc8ffff);
     env->vfp.vec_len = (val >> 16) & 7;