armv7m: MRS/MSR: handle unprivileged access

The MRS and MSR instruction handling has a number of flaws:
* unprivileged accesses should only be able to read
CONTROL and the xPSR subfields, and only write APSR
(others RAZ/WI)
* privileged access should not be able to write xPSR
subfields other than APSR
* accesses to unimplemented registers should log as
guest errors, not abort QEMU

Backports commit 58117c9bb429cd9552d998687aa99088eb1d8528 from qemu

qemu/target/arm/helper.c

@@ -7627,23 +7627,39 @@ hwaddr arm_cpu_get_phys_page_attrs_debug(CPUState *cs, vaddr addr,
 
 uint32_t HELPER(v7m_mrs)(CPUARMState *env, uint32_t reg)
 {
-    ARMCPU *cpu = arm_env_get_cpu(env);
+    uint32_t mask;
+    unsigned el = arm_current_el(env);
+
+    /* First handle registers which unprivileged can read */
+
+    switch (reg) {
+    case 0:
+    case 1:
+    case 2:
+    case 3:
+    case 4:
+    case 5:
+    case 6:
+    case 7: /* xPSR sub-fields */
+        mask = 0;
+        if ((reg & 1) && el) {
+            mask |= 0x000001ff; /* IPSR (unpriv. reads as zero) */
+        }
+        if (!(reg & 4)) {
+            mask |= 0xf8000000; /* APSR */
+        }
+        /* EPSR reads as zero */
+        return xpsr_read(env) & mask;
+        break;
+    case 20: /* CONTROL */
+        return env->v7m.control;
+    }
+
+    if (el == 0) {
+        return 0; /* unprivileged reads others as zero */
+    }
 
     switch (reg) {
-    case 0: /* APSR */
-        return xpsr_read(env) & 0xf8000000;
-    case 1: /* IAPSR */
-        return xpsr_read(env) & 0xf80001ff;
-    case 2: /* EAPSR */
-        return xpsr_read(env) & 0xff00fc00;
-    case 3: /* xPSR */
-        return xpsr_read(env) & 0xff00fdff;
-    case 5: /* IPSR */
-        return xpsr_read(env) & 0x000001ff;
-    case 6: /* EPSR */
-        return xpsr_read(env) & 0x0700fc00;
-    case 7: /* IEPSR */
-        return xpsr_read(env) & 0x0700edff;
     case 8: /* MSP */
         return (env->v7m.control & R_V7M_CONTROL_SPSEL_MASK) ?
             env->v7m.other_sp : env->regs[13];
@@ -7657,40 +7673,34 @@ uint32_t HELPER(v7m_mrs)(CPUARMState *env, uint32_t reg)
         return env->v7m.basepri;
     case 19: /* FAULTMASK */
         return (env->daif & PSTATE_F) != 0;
-    case 20: /* CONTROL */
-        return env->v7m.control;
     default:
         /* ??? For debugging only.  */
-        cpu_abort(CPU(cpu), "Unimplemented system register read (%d)\n", reg);
+        qemu_log_mask(LOG_GUEST_ERROR, "Attempt to read unknown special"
+                                       " register %d\n", reg);
         return 0;
     }
 }
 
 void HELPER(v7m_msr)(CPUARMState *env, uint32_t reg, uint32_t val)
 {
-    ARMCPU *cpu = arm_env_get_cpu(env);
+    if (arm_current_el(env) == 0 && reg > 7) {
+        /* only xPSR sub-fields may be written by unprivileged */
+        return;
+    }
 
     switch (reg) {
-    case 0: /* APSR */
+    case 0:
-        xpsr_write(env, val, 0xf8000000);
+    case 1:
-        break;
+    case 2:
-    case 1: /* IAPSR */
+    case 3:
-        xpsr_write(env, val, 0xf8000000);
+    case 4:
-        break;
+    case 5:
-    case 2: /* EAPSR */
+    case 6:
-        xpsr_write(env, val, 0xfe00fc00);
+    case 7: /* xPSR sub-fields */
-        break;
+        /* only APSR is actually writable */
-    case 3: /* xPSR */
+        if (reg & 4) {
-        xpsr_write(env, val, 0xfe00fc00);
+            xpsr_write(env, val, 0xf8000000); /* APSR */
-        break;
+        }
-    case 5: /* IPSR */
-        /* IPSR bits are readonly.  */
-        break;
-    case 6: /* EPSR */
-        xpsr_write(env, val, 0x0600fc00);
-        break;
-    case 7: /* IEPSR */
-        xpsr_write(env, val, 0x0600fc00);
         break;
     case 8: /* MSP */
         if (env->v7m.control & R_V7M_CONTROL_SPSEL_MASK) {
@@ -7734,8 +7744,8 @@ void HELPER(v7m_msr)(CPUARMState *env, uint32_t reg, uint32_t val)
                                   R_V7M_CONTROL_NPRIV_MASK);
         break;
     default:
-        /* ??? For debugging only.  */
+        qemu_log_mask(LOG_GUEST_ERROR, "Attempt to write unknown special"
-        cpu_abort(CPU(cpu), "Unimplemented system register write (%d)\n", reg);
+                                       " register %d\n", reg);
         return;
     }
 }