target-arm: Properly support EL2 and EL3 in arm_el_is_aa64()

Support EL2 and EL3 in arm_el_is_aa64() by implementing the
logic for checking the SCR_EL3 and HCR_EL2 register-width bits
as appropriate to determine the register width of lower exception
levels.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>

target-arm/cpu.h

@@ -969,18 +969,33 @@ static inline bool arm_is_secure(CPUARMState *env)
 /* Return true if the specified exception level is running in AArch64 state. */
 static inline bool arm_el_is_aa64(CPUARMState *env, int el)
 {
-    /* We don't currently support EL2, and this isn't valid for EL0
-     * (if we're in EL0, is_a64() is what you want, and if we're not in EL0
-     * then the state of EL0 isn't well defined.)
+    /* This isn't valid for EL0 (if we're in EL0, is_a64() is what you want,
+     * and if we're not in EL0 then the state of EL0 isn't well defined.)
      */
-    assert(el == 1 || el == 3);
+    assert(el >= 1 && el <= 3);
+    bool aa64 = arm_feature(env, ARM_FEATURE_AARCH64);
 
-    /* AArch64-capable CPUs always run with EL1 in AArch64 mode. This
-     * is a QEMU-imposed simplification which we may wish to change later.
-     * If we in future support EL2 and/or EL3, then the state of lower
-     * exception levels is controlled by the HCR.RW and SCR.RW bits.
+    /* The highest exception level is always at the maximum supported
+     * register width, and then lower levels have a register width controlled
+     * by bits in the SCR or HCR registers.
      */
-    return arm_feature(env, ARM_FEATURE_AARCH64);
+    if (el == 3) {
+        return aa64;
+    }
+
+    if (arm_feature(env, ARM_FEATURE_EL3)) {
+        aa64 = aa64 && (env->cp15.scr_el3 & SCR_RW);
+    }
+
+    if (el == 2) {
+        return aa64;
+    }
+
+    if (arm_feature(env, ARM_FEATURE_EL2) && !arm_is_secure_below_el3(env)) {
+        aa64 = aa64 && (env->cp15.hcr_el2 & HCR_RW);
+    }
+
+    return aa64;
 }
 
 /* Function for determing whether guest cp register reads and writes should