target/arm: Allow M-profile CPUs with FP16 to set FPSCR.FP16

M-profile CPUs with half-precision floating point support should
be able to write to FPSCR.FZ16, but an M-profile specific masking
of the value at the top of vfp_set_fpscr() currently prevents that.
This is not yet an active bug because we have no M-profile
FP16 CPUs, but needs to be fixed before we can add any.

The bits that the masking is effectively preventing from being
set are the A-profile only short-vector Len and Stride fields,
plus the Neon QC bit. Rearrange the order of the function so
that those fields are handled earlier and only under a suitable
guard; this allows us to drop the M-profile specific masking,
making FZ16 writeable.

This change also makes the QC bit correctly RAZ/WI for older
no-Neon A-profile cores.

This refactoring also paves the way for the low-overhead-branch
LTPSIZE field, which uses some of the bits that are used for
A-profile Stride and Len.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201019151301.2046-10-peter.maydell@linaro.org

1 files changed, 28 insertions, 19 deletions

diff --git a/target/arm/vfp_helper.c b/target/arm/vfp_helper.c
index abfdb6a8e2..364856498c 100644
--- a/target/arm/vfp_helper.c
+++ b/target/arm/vfp_helper.c
@@ -194,36 +194,45 @@ void HELPER(vfp_set_fpscr)(CPUARMState *env, uint32_t val)
         val &= ~FPCR_FZ16;
     }
 
-    if (arm_feature(env, ARM_FEATURE_M)) {
+    vfp_set_fpscr_to_host(env, val);
+
+    if (!arm_feature(env, ARM_FEATURE_M)) {
         /*
-         * M profile FPSCR is RES0 for the QC, STRIDE, FZ16, LEN bits
-         * and also for the trapped-exception-handling bits IxE.
+         * Short-vector length and stride; on M-profile these bits
+         * are used for different purposes.
+         * We can't make this conditional be "if MVFR0.FPShVec != 0",
+         * because in v7A no-short-vector-support cores still had to
+         * allow Stride/Len to be written with the only effect that
+         * some insns are required to UNDEF if the guest sets them.
+         *
+         * TODO: if M-profile MVE implemented, set LTPSIZE.
          */
-        val &= 0xf7c0009f;
+        env->vfp.vec_len = extract32(val, 16, 3);
+        env->vfp.vec_stride = extract32(val, 20, 2);
     }
 
-    vfp_set_fpscr_to_host(env, val);
+    if (arm_feature(env, ARM_FEATURE_NEON)) {
+        /*
+         * The bit we set within fpscr_q is arbitrary; the register as a
+         * whole being zero/non-zero is what counts.
+         * TODO: M-profile MVE also has a QC bit.
+         */
+        env->vfp.qc[0] = val & FPCR_QC;
+        env->vfp.qc[1] = 0;
+        env->vfp.qc[2] = 0;
+        env->vfp.qc[3] = 0;
+    }
 
     /*
      * We don't implement trapped exception handling, so the
      * trap enable bits, IDE|IXE|UFE|OFE|DZE|IOE are all RAZ/WI (not RES0!)
      *
-     * If we exclude the exception flags, IOC|DZC|OFC|UFC|IXC|IDC
-     * (which are stored in fp_status), and the other RES0 bits
-     * in between, then we clear all of the low 16 bits.
+     * The exception flags IOC|DZC|OFC|UFC|IXC|IDC are stored in
+     * fp_status; QC, Len and Stride are stored separately earlier.
+     * Clear out all of those and the RES0 bits: only NZCV, AHP, DN,
+     * FZ, RMode and FZ16 are kept in vfp.xregs[FPSCR].
      */
     env->vfp.xregs[ARM_VFP_FPSCR] = val & 0xf7c80000;
-    env->vfp.vec_len = (val >> 16) & 7;
-    env->vfp.vec_stride = (val >> 20) & 3;
-
-    /*
-     * The bit we set within fpscr_q is arbitrary; the register as a
-     * whole being zero/non-zero is what counts.
-     */
-    env->vfp.qc[0] = val & FPCR_QC;
-    env->vfp.qc[1] = 0;
-    env->vfp.qc[2] = 0;
-    env->vfp.qc[3] = 0;
 }
 
 void vfp_set_fpscr(CPUARMState *env, uint32_t val)