target/arm: For v8.1M, always clear R0-R3, R12, APSR, EPSR on excepti…

…on entry

In v8.0M, on exception entry the registers R0-R3, R12, APSR and EPSR
are zeroed for an exception taken to Non-secure state; for an
exception taken to Secure state they become UNKNOWN, and we chose to
leave them at their previous values.

In v8.1M the behaviour is specified more tightly and these registers
are always zeroed regardless of the security state that the exception
targets (see rule R_KPZV).  Implement this.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201119215617.29887-17-peter.maydell@linaro.org

target/arm/m_helper.c

@@ -897,10 +897,12 @@ static void v7m_exception_taken(ARMCPU *cpu, uint32_t lr, bool dotailchain,
          * Clear registers if necessary to prevent non-secure exception
          * code being able to see register values from secure code.
          * Where register values become architecturally UNKNOWN we leave
-         * them with their previous values.
+         * them with their previous values. v8.1M is tighter than v8.0M
+         * here and always zeroes the caller-saved registers regardless
+         * of the security state the exception is targeting.
          */
         if (arm_feature(env, ARM_FEATURE_M_SECURITY)) {
-            if (!targets_secure) {
+            if (!targets_secure || arm_feature(env, ARM_FEATURE_V8_1M)) {
                 /*
                  * Always clear the caller-saved registers (they have been
                  * pushed to the stack earlier in v7m_push_stack()).
@@ -909,10 +911,16 @@ static void v7m_exception_taken(ARMCPU *cpu, uint32_t lr, bool dotailchain,
                  * v7m_push_callee_stack()).
                  */
                 int i;
+                /*
+                 * r4..r11 are callee-saves, zero only if background
+                 * state was Secure (EXCRET.S == 1) and exception
+                 * targets Non-secure state
+                 */
+                bool zero_callee_saves = !targets_secure &&
+                    (lr & R_V7M_EXCRET_S_MASK);
 
                 for (i = 0; i < 13; i++) {
-                    /* r4..r11 are callee-saves, zero only if EXCRET.S == 1 */
-                    if (i < 4 || i > 11 || (lr & R_V7M_EXCRET_S_MASK)) {
+                    if (i < 4 || i > 11 || zero_callee_saves) {
                         env->regs[i] = 0;
                     }
                 }