target/arm: Fix SCR RES1 handling

The FW and AW bits of SCR_EL3 are RES1 only in some contexts. Force them
to 1 only when there is no support for AArch32 at EL1 or above.

The reset value will be 0x30 only if the CPU is AArch64-only; if there
is support for AArch32 at EL1 or above, it will be reset to 0.

Also adds helper function isar_feature_aa64_aa32_el1 to check if AArch32
is supported at EL1 or above.

Signed-off-by: Mike Nawrocki <michael.nawrocki@gtri.gatech.edu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210203165552.16306-2-michael.nawrocki@gtri.gatech.edu
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/cpu.h

@@ -4033,6 +4033,11 @@ static inline bool isar_feature_aa64_aa32(const ARMISARegisters *id)
     return FIELD_EX64(id->id_aa64pfr0, ID_AA64PFR0, EL0) >= 2;
 }
 
+static inline bool isar_feature_aa64_aa32_el1(const ARMISARegisters *id)
+{
+    return FIELD_EX64(id->id_aa64pfr0, ID_AA64PFR0, EL1) >= 2;
+}
+
 static inline bool isar_feature_aa64_sve(const ARMISARegisters *id)
 {
     return FIELD_EX64(id->id_aa64pfr0, ID_AA64PFR0, SVE) != 0;

target/arm/helper.c

@@ -2024,7 +2024,10 @@ static void scr_write(CPUARMState *env, const ARMCPRegInfo *ri, uint64_t value)
     ARMCPU *cpu = env_archcpu(env);
 
     if (ri->state == ARM_CP_STATE_AA64) {
-        value |= SCR_FW | SCR_AW;   /* these two bits are RES1.  */
+        if (arm_feature(env, ARM_FEATURE_AARCH64) &&
+            !cpu_isar_feature(aa64_aa32_el1, cpu)) {
+                value |= SCR_FW | SCR_AW;   /* these two bits are RES1.  */
+        }
         valid_mask &= ~SCR_NET;
 
         if (cpu_isar_feature(aa64_lor, cpu)) {
@@ -2063,6 +2066,15 @@ static void scr_write(CPUARMState *env, const ARMCPRegInfo *ri, uint64_t value)
     raw_write(env, ri, value);
 }
 
+static void scr_reset(CPUARMState *env, const ARMCPRegInfo *ri)
+{
+    /*
+     * scr_write will set the RES1 bits on an AArch64-only CPU.
+     * The reset value will be 0x30 on an AArch64-only CPU and 0 otherwise.
+     */
+    scr_write(env, ri, 0);
+}
+
 static CPAccessResult access_aa64_tid2(CPUARMState *env,
                                        const ARMCPRegInfo *ri,
                                        bool isread)
@@ -5785,7 +5797,7 @@ static const ARMCPRegInfo el3_cp_reginfo[] = {
     { .name = "SCR_EL3", .state = ARM_CP_STATE_AA64,
       .opc0 = 3, .opc1 = 6, .crn = 1, .crm = 1, .opc2 = 0,
       .access = PL3_RW, .fieldoffset = offsetof(CPUARMState, cp15.scr_el3),
-      .resetvalue = 0, .writefn = scr_write },
+      .resetfn = scr_reset, .writefn = scr_write },
     { .name = "SCR",  .type = ARM_CP_ALIAS | ARM_CP_NEWEL,
       .cp = 15, .opc1 = 0, .crn = 1, .crm = 1, .opc2 = 0,
       .access = PL1_RW, .accessfn = access_trap_aa32s_el1,