hw/intc/armv7m_nvic: Update FPDSCR masking for v8.1M

The FPDSCR register has a similar layout to the FPSCR.  In v8.1M it
gains new fields FZ16 (if half-precision floating point is supported)
and LTPSIZE (always reads as 4).  Update the reset value and the code
that handles writes to this register accordingly.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201119215617.29887-16-peter.maydell@linaro.org

hw/intc/armv7m_nvic.c

@@ -2068,7 +2068,14 @@ static void nvic_writel(NVICState *s, uint32_t offset, uint32_t value,
         break;
     case 0xf3c: /* FPDSCR */
         if (cpu_isar_feature(aa32_vfp_simd, cpu)) {
-            value &= 0x07c00000;
+            uint32_t mask = FPCR_AHP | FPCR_DN | FPCR_FZ | FPCR_RMODE_MASK;
+            if (cpu_isar_feature(any_fp16, cpu)) {
+                mask |= FPCR_FZ16;
+            }
+            value &= mask;
+            if (cpu_isar_feature(aa32_lob, cpu)) {
+                value |= 4 << FPCR_LTPSIZE_SHIFT;
+            }
             cpu->env.v7m.fpdscr[attrs.secure] = value;
         }
         break;

target/arm/cpu.c

@@ -262,6 +262,9 @@ static void arm_cpu_reset(DeviceState *dev)
              * always reset to 4.
              */
             env->v7m.ltpsize = 4;
+            /* The LTPSIZE field in FPDSCR is constant and reads as 4. */
+            env->v7m.fpdscr[M_REG_NS] = 4 << FPCR_LTPSIZE_SHIFT;
+            env->v7m.fpdscr[M_REG_S] = 4 << FPCR_LTPSIZE_SHIFT;
         }
 
         if (arm_feature(env, ARM_FEATURE_M_SECURITY)) {

target/arm/cpu.h

@@ -1521,14 +1521,19 @@ void vfp_set_fpscr(CPUARMState *env, uint32_t val);
 #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_RMODE_MASK (3 << 22) /* Rounding mode */
 #define FPCR_FZ     (1 << 24)   /* Flush-to-zero enable bit */
 #define FPCR_DN     (1 << 25)   /* Default NaN enable bit */
+#define FPCR_AHP    (1 << 26)   /* Alternative half-precision */
 #define FPCR_QC     (1 << 27)   /* Cumulative saturation bit */
 #define FPCR_V      (1 << 28)   /* FP overflow flag */
 #define FPCR_C      (1 << 29)   /* FP carry flag */
 #define FPCR_Z      (1 << 30)   /* FP zero flag */
 #define FPCR_N      (1 << 31)   /* FP negative flag */
 
+#define FPCR_LTPSIZE_SHIFT 16   /* LTPSIZE, M-profile only */
+#define FPCR_LTPSIZE_MASK (7 << FPCR_LTPSIZE_SHIFT)
+
 #define FPCR_NZCV_MASK (FPCR_N | FPCR_Z | FPCR_C | FPCR_V)
 #define FPCR_NZCVQC_MASK (FPCR_NZCV_MASK | FPCR_QC)