target/arm: Honour M-profile FP enable bits

Like AArch64, M-profile floating point has no FPEXC enable
bit to gate floating point; so always set the VFPEN TB flag.

M-profile also has CPACR and NSACR similar to A-profile;
they behave slightly differently:
 * the CPACR is banked between Secure and Non-Secure
 * if the NSACR forces a trap then this is taken to
   the Secure state, not the Non-Secure state

Honour the CPACR and NSACR settings. The NSACR handling
requires us to borrow the exception.target_el field
(usually meaningless for M profile) to distinguish the
NOCP UsageFault taken to Secure state from the more
usual fault taken to the current security state.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190416125744.27770-6-peter.maydell@linaro.org

target/arm/helper.c

@@ -7556,6 +7556,25 @@ uint32_t arm_phys_excp_target_el(CPUState *cs, uint32_t excp_idx,
     return target_el;
 }
 
+/*
+ * Return true if the v7M CPACR permits access to the FPU for the specified
+ * security state and privilege level.
+ */
+static bool v7m_cpacr_pass(CPUARMState *env, bool is_secure, bool is_priv)
+{
+    switch (extract32(env->v7m.cpacr[is_secure], 20, 2)) {
+    case 0:
+    case 2: /* UNPREDICTABLE: we treat like 0 */
+        return false;
+    case 1:
+        return is_priv;
+    case 3:
+        return true;
+    default:
+        g_assert_not_reached();
+    }
+}
+
 static bool v7m_stack_write(ARMCPU *cpu, uint32_t addr, uint32_t value,
                             ARMMMUIdx mmu_idx, bool ignfault)
 {
@@ -8815,9 +8834,23 @@ void arm_v7m_cpu_do_interrupt(CPUState *cs)
         env->v7m.cfsr[env->v7m.secure] |= R_V7M_CFSR_UNDEFINSTR_MASK;
         break;
     case EXCP_NOCP:
-        armv7m_nvic_set_pending(env->nvic, ARMV7M_EXCP_USAGE, env->v7m.secure);
-        env->v7m.cfsr[env->v7m.secure] |= R_V7M_CFSR_NOCP_MASK;
+    {
+        /*
+         * NOCP might be directed to something other than the current
+         * security state if this fault is because of NSACR; we indicate
+         * the target security state using exception.target_el.
+         */
+        int target_secstate;
+
+        if (env->exception.target_el == 3) {
+            target_secstate = M_REG_S;
+        } else {
+            target_secstate = env->v7m.secure;
+        }
+        armv7m_nvic_set_pending(env->nvic, ARMV7M_EXCP_USAGE, target_secstate);
+        env->v7m.cfsr[target_secstate] |= R_V7M_CFSR_NOCP_MASK;
         break;
+    }
     case EXCP_INVSTATE:
         armv7m_nvic_set_pending(env->nvic, ARMV7M_EXCP_USAGE, env->v7m.secure);
         env->v7m.cfsr[env->v7m.secure] |= R_V7M_CFSR_INVSTATE_MASK;
@@ -12751,6 +12784,22 @@ int fp_exception_el(CPUARMState *env, int cur_el)
         return 0;
     }
 
+    if (arm_feature(env, ARM_FEATURE_M)) {
+        /* CPACR can cause a NOCP UsageFault taken to current security state */
+        if (!v7m_cpacr_pass(env, env->v7m.secure, cur_el != 0)) {
+            return 1;
+        }
+
+        if (arm_feature(env, ARM_FEATURE_M_SECURITY) && !env->v7m.secure) {
+            if (!extract32(env->v7m.nsacr, 10, 1)) {
+                /* FP insns cause a NOCP UsageFault taken to Secure */
+                return 3;
+            }
+        }
+
+        return 0;
+    }
+
     /* The CPACR controls traps to EL1, or PL1 if we're 32 bit:
      * 0, 2 : trap EL0 and EL1/PL1 accesses
      * 1    : trap only EL0 accesses
@@ -12938,7 +12987,7 @@ void cpu_get_tb_cpu_state(CPUARMState *env, target_ulong *pc,
         flags = FIELD_DP32(flags, TBFLAG_A32, SCTLR_B, arm_sctlr_b(env));
         flags = FIELD_DP32(flags, TBFLAG_A32, NS, !access_secure_reg(env));
         if (env->vfp.xregs[ARM_VFP_FPEXC] & (1 << 30)
-            || arm_el_is_aa64(env, 1)) {
+            || arm_el_is_aa64(env, 1) || arm_feature(env, ARM_FEATURE_M)) {
             flags = FIELD_DP32(flags, TBFLAG_A32, VFPEN, 1);
         }
         flags = FIELD_DP32(flags, TBFLAG_A32, XSCALE_CPAR, env->cp15.c15_cpar);

target/arm/translate.c

@@ -3399,8 +3399,14 @@ static int disas_vfp_insn(DisasContext *s, uint32_t insn)
      * for attempts to execute invalid vfp/neon encodings with FP disabled.
      */
     if (s->fp_excp_el) {
-        gen_exception_insn(s, 4, EXCP_UDEF,
-                           syn_fp_access_trap(1, 0xe, false), s->fp_excp_el);
+        if (arm_dc_feature(s, ARM_FEATURE_M)) {
+            gen_exception_insn(s, 4, EXCP_NOCP, syn_uncategorized(),
+                               s->fp_excp_el);
+        } else {
+            gen_exception_insn(s, 4, EXCP_UDEF,
+                               syn_fp_access_trap(1, 0xe, false),
+                               s->fp_excp_el);
+        }
         return 0;
     }