arm: add MPU support to M profile CPUs

The M series MPU is almost the same as the already implemented R
profile MPU (v7 PMSA). So all we need to implement here is the MPU
register interface in the system register space.

This implementation has the same restriction as the R profile MPU
that it doesn't permit regions to be sized down smaller than 1K.

We also do not yet implement support for MPU_CTRL.HFNMIENA; this
bit should if zero disable use of the MPU when running HardFault,
NMI or with FAULTMASK set to 1 (ie at an execution priority of
less than zero) -- if the MPU is enabled we don't treat these
cases any differently.

Backports commit 29c483a506070e8f554c77d22686f405e30b9114 from qemu

qemu/target/arm/cpu.h

@@ -424,6 +424,7 @@ typedef struct CPUARMState {
         uint32_t dfsr; /* Debug Fault Status Register */
         uint32_t mmfar; /* MemManage Fault Address */
         uint32_t bfar; /* BusFault Address */
+        unsigned mpu_ctrl; /* MPU_CTRL (some bits kept in sctlr_el[1]) */
         int exception;
         uint32_t secure; /* Is CPU in Secure state? (not guest visible) */
     } v7m;
@@ -1149,6 +1150,11 @@ FIELD(V7M_DFSR, DWTTRAP, 2, 1)
 FIELD(V7M_DFSR, VCATCH, 3, 1)
 FIELD(V7M_DFSR, EXTERNAL, 4, 1)
 
+/* v7M MPU_CTRL bits */
+FIELD(V7M_MPU_CTRL, ENABLE, 0, 1)
+FIELD(V7M_MPU_CTRL, HFNMIENA, 1, 1)
+FIELD(V7M_MPU_CTRL, PRIVDEFENA, 2, 1)
+
 /* If adding a feature bit which corresponds to a Linux ELF
  * HWCAP bit, remember to update the feature-bit-to-hwcap
  * mapping in linux-user/elfload.c:get_elf_hwcap().

qemu/target/arm/helper.c

@@ -6227,6 +6227,10 @@ static inline uint32_t regime_sctlr(CPUARMState *env, ARMMMUIdx mmu_idx)
 static inline bool regime_translation_disabled(CPUARMState *env,
                                                ARMMMUIdx mmu_idx)
 {
+    if (arm_feature(env, ARM_FEATURE_M)) {
+        return !(env->v7m.mpu_ctrl & R_V7M_MPU_CTRL_ENABLE_MASK);
+    }
+
     if (mmu_idx == ARMMMUIdx_S2NS) {
         return (env->cp15.hcr_el2 & HCR_VM) == 0;
     }
@@ -7352,6 +7356,25 @@ static inline void get_phys_addr_pmsav7_default(CPUARMState *env,
     }
 }
 
+static bool pmsav7_use_background_region(ARMCPU *cpu,
+                                         ARMMMUIdx mmu_idx, bool is_user)
+{
+    /* Return true if we should use the default memory map as a
+     * "background" region if there are no hits against any MPU regions.
+     */
+    CPUARMState *env = &cpu->env;
+
+    if (is_user) {
+        return false;
+    }
+
+    if (arm_feature(env, ARM_FEATURE_M)) {
+        return env->v7m.mpu_ctrl & R_V7M_MPU_CTRL_PRIVDEFENA_MASK;
+    } else {
+        return regime_sctlr(env, mmu_idx) & SCTLR_BR;
+    }
+}
+
 static bool get_phys_addr_pmsav7(CPUARMState *env, uint32_t address,
                                  int access_type, ARMMMUIdx mmu_idx,
                                  hwaddr *phys_ptr, int *prot, uint32_t *fsr)
@@ -7439,7 +7462,7 @@ static bool get_phys_addr_pmsav7(CPUARMState *env, uint32_t address,
         }
 
         if (n == -1) { /* no hits */
-            if (is_user || !(regime_sctlr(env, mmu_idx) & SCTLR_BR)) {
+            if (!pmsav7_use_background_region(cpu, mmu_idx, is_user)) {
                 /* background fault */
                 *fsr = 0;
                 return true;