[PATCH 09/10] target/arm: Implement FPSCR.LTPSIZE for M-profile LOB extension

[Peter Maydell]

If the M-profile low-overhead-branch extension is implemented, FPSCR
bits [18:16] are a new field LTPSIZE.  If MVE is not implemented
(currently always true for us) then this field always reads as 4 and
ignores writes.

These bits used to be the vector-length field for the old
short-vector extension, so we need to take care that they
are not misinterpreted as setting vec_len.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 target/arm/cpu.c        |  5 +++++
 target/arm/vfp_helper.c | 25 +++++++++++++++++++++----
 2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/target/arm/cpu.c b/target/arm/cpu.c
index 186ee621a65..baae826f94f 100644
--- a/target/arm/cpu.c
+++ b/target/arm/cpu.c
@@ -255,6 +255,11 @@ static void arm_cpu_reset(DeviceState *dev)
         uint8_t *rom;
         uint32_t vecbase;
 
+        if (cpu_isar_feature(aa32_lob, cpu)) {
+            /* LTPSIZE is constant 4 if MVE not implemented */
+            env->vfp.xregs[ARM_VFP_FPSCR] |= 4 << 16;
+        }
+
         if (arm_feature(env, ARM_FEATURE_M_SECURITY)) {
             env->v7m.secure = true;
         } else {
diff --git a/target/arm/vfp_helper.c b/target/arm/vfp_helper.c
index 5666393ef79..350150adbf1 100644
--- a/target/arm/vfp_helper.c
+++ b/target/arm/vfp_helper.c
@@ -189,8 +189,10 @@ uint32_t vfp_get_fpscr(CPUARMState *env)
 
 void HELPER(vfp_set_fpscr)(CPUARMState *env, uint32_t val)
 {
+    ARMCPU *cpu = env_archcpu(env);
+
     /* When ARMv8.2-FP16 is not supported, FZ16 is RES0.  */
-    if (!cpu_isar_feature(any_fp16, env_archcpu(env))) {
+    if (!cpu_isar_feature(any_fp16, cpu)) {
         val &= ~FPCR_FZ16;
     }
 
@@ -198,8 +200,14 @@ void HELPER(vfp_set_fpscr)(CPUARMState *env, uint32_t val)
         /*
          * M profile FPSCR is RES0 for the QC, STRIDE, FZ16, LEN bits
          * and also for the trapped-exception-handling bits IxE.
+         * From v8.1M with the low-overhead-loop extension bits
+         * [18:16] are used for LTPSIZE and (since we don't implement
+         * MVE) always read as 4 and ignore writes.
          */
         val &= 0xf7c0009f;
+        if (cpu_isar_feature(aa32_lob, cpu)) {
+            val |= 4 << 16;
+        }
     }
 
     vfp_set_fpscr_to_host(env, val);
@@ -212,9 +220,18 @@ void HELPER(vfp_set_fpscr)(CPUARMState *env, uint32_t val)
      * (which are stored in fp_status), and the other RES0 bits
      * in between, then we clear all of the low 16 bits.
      */
-    env->vfp.xregs[ARM_VFP_FPSCR] = val & 0xf7c80000;
-    env->vfp.vec_len = (val >> 16) & 7;
-    env->vfp.vec_stride = (val >> 20) & 3;
+    if (cpu_isar_feature(aa32_lob, cpu)) {
+        /*
+         * M-profile low-overhead-loop extension: [18:16] are LTPSIZE
+         * and we keep them in vfp.xregs[].
+         */
+        env->vfp.xregs[ARM_VFP_FPSCR] = val & 0xf7cf0000;
+    } else {
+        /* Those bits might be the old-style short vector length/stride */
+        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
-- 
2.20.1