target/arm: trap DCC access in user mode emulation

Accessing EL0-accessible Debug Communication Channel (DCC) registers in
user mode emulation is currently enabled.  However, it does not match
Linux behavior as Linux sets MDSCR_EL1.TDCC on startup to disable EL0
access to DCC (see __cpu_setup() in arch/arm64/mm/proc.S).

This patch fixes access_tdcc() to check MDSCR_EL1.TDCC for EL0 and sets
MDSCR_EL1.TDCC for user mode emulation to match Linux.

Signed-off-by: Zhuojia Shen <chaosdefinition@hotmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: DS7PR12MB630905198DD8E69F6817544CAC4EA@DS7PR12MB6309.namprd12.prod.outlook.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/cpu.c

@@ -289,6 +289,8 @@ static void arm_cpu_reset_hold(Object *obj)
          * This is not yet exposed from the Linux kernel in any way.
          */
         env->cp15.sctlr_el[1] |= SCTLR_TSCXT;
+        /* Disable access to Debug Communication Channel (DCC). */
+        env->cp15.mdscr_el1 |= 1 << 12;
 #else
         /* Reset into the highest available EL */
         if (arm_feature(env, ARM_FEATURE_EL3)) {

target/arm/debug_helper.c

@@ -842,12 +842,14 @@ static CPAccessResult access_tda(CPUARMState *env, const ARMCPRegInfo *ri,
  * is implemented then these are controlled by MDCR_EL2.TDCC for
  * EL2 and MDCR_EL3.TDCC for EL3. They are also controlled by
  * the general debug access trap bits MDCR_EL2.TDA and MDCR_EL3.TDA.
+ * For EL0, they are also controlled by MDSCR_EL1.TDCC.
  */
 static CPAccessResult access_tdcc(CPUARMState *env, const ARMCPRegInfo *ri,
                                   bool isread)
 {
     int el = arm_current_el(env);
     uint64_t mdcr_el2 = arm_mdcr_el2_eff(env);
+    bool mdscr_el1_tdcc = extract32(env->cp15.mdscr_el1, 12, 1);
     bool mdcr_el2_tda = (mdcr_el2 & MDCR_TDA) || (mdcr_el2 & MDCR_TDE) ||
         (arm_hcr_el2_eff(env) & HCR_TGE);
     bool mdcr_el2_tdcc = cpu_isar_feature(aa64_fgt, env_archcpu(env)) &&
@@ -855,6 +857,9 @@ static CPAccessResult access_tdcc(CPUARMState *env, const ARMCPRegInfo *ri,
     bool mdcr_el3_tdcc = cpu_isar_feature(aa64_fgt, env_archcpu(env)) &&
                                           (env->cp15.mdcr_el3 & MDCR_TDCC);
 
+    if (el < 1 && mdscr_el1_tdcc) {
+        return CP_ACCESS_TRAP;
+    }
     if (el < 2 && (mdcr_el2_tda || mdcr_el2_tdcc)) {
         return CP_ACCESS_TRAP_EL2;
     }