[Qemu-devel] [PATCH v2 22/26] armv7m: priority field mask

[Michael Davidsaver]

Many v7m CPUs don't implement all of the 8 bits
of the priority fields.  Typically, only the
top N bits are available.  Existing practice
implies that writes to unimplemented
bits will be ignore, and read as zero.

This allows a guest to discover the implemented
bits by writing 0xff to (eg. basepri).
The value read back will have only the available
bits set.
---
 hw/intc/armv7m_nvic.c | 2 ++
 target-arm/cpu-qom.h  | 6 ++++++
 target-arm/cpu.c      | 7 +++++++
 target-arm/helper.c   | 6 ++++--
 4 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/hw/intc/armv7m_nvic.c b/hw/intc/armv7m_nvic.c
index 2b42d9d..e2410a3 100644
--- a/hw/intc/armv7m_nvic.c
+++ b/hw/intc/armv7m_nvic.c
@@ -149,6 +149,8 @@ void set_prio(NVICState *s, unsigned irq, uint8_t prio)
     assert(irq > 3); /* only use for configurable prios */
     assert(irq < NVIC_MAX_VECTORS);
 
+    prio &= s->cpu->v7m_priority_mask;
+
     s->vectors[irq].raw_prio = prio;
     s->vectors[irq].prio_group = (prio>>(s->prigroup+1));
     s->vectors[irq].prio_sub = irq + (prio & submask) * NVIC_MAX_VECTORS;
diff --git a/target-arm/cpu-qom.h b/target-arm/cpu-qom.h
index 25fb1ce..79cf591 100644
--- a/target-arm/cpu-qom.h
+++ b/target-arm/cpu-qom.h
@@ -108,6 +108,12 @@ typedef struct ARMCPU {
     /* PMSAv7 MPU number of supported regions */
     uint32_t pmsav7_dregion;
 
+    /* Some v7-M targets don't impliment the full 8 bits
+     * of the priority fields.  Writes to unimplimented
+     * bits are treated as zero (guest can discover mask).
+     */
+    uint8_t v7m_priority_mask;
+
     /* PSCI conduit used to invoke PSCI methods
      * 0 - disabled, 1 - smc, 2 - hvc
      */
diff --git a/target-arm/cpu.c b/target-arm/cpu.c
index 8b85888..27cf482 100644
--- a/target-arm/cpu.c
+++ b/target-arm/cpu.c
@@ -527,6 +527,9 @@ static Property arm_cpu_has_mpu_property =
 static Property arm_cpu_pmsav7_dregion_property =
             DEFINE_PROP_UINT32("pmsav7-dregion", ARMCPU, pmsav7_dregion, 16);
 
+static Property armv7m_priority_mask_property =
+        DEFINE_PROP_UINT8("priority-mask", ARMCPU, v7m_priority_mask, 0xff);
+
 static void arm_cpu_post_init(Object *obj)
 {
     ARMCPU *cpu = ARM_CPU(obj);
@@ -565,6 +568,10 @@ static void arm_cpu_post_init(Object *obj)
         }
     }
 
+    if (IS_M(&cpu->env) && arm_feature(&cpu->env, ARM_FEATURE_V7)) {
+        qdev_property_add_static(DEVICE(obj), &armv7m_priority_mask_property,
+                                 &error_abort);
+    }
 }
 
 static void arm_cpu_finalizefn(Object *obj)
diff --git a/target-arm/helper.c b/target-arm/helper.c
index 2661da4..c890b3a 100644
--- a/target-arm/helper.c
+++ b/target-arm/helper.c
@@ -7569,6 +7569,8 @@ uint32_t HELPER(v7m_mrs)(CPUARMState *env, uint32_t reg)
 
 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;
@@ -7601,10 +7603,10 @@ void HELPER(v7m_msr)(CPUARMState *env, uint32_t reg, 
uint32_t val)
         }
         break;
     case 17: /* BASEPRI */
-        env->v7m.basepri = val & 0xff;
+        env->v7m.basepri = val & cpu->v7m_priority_mask;
         break;
     case 18: /* BASEPRI_MAX */
-        val &= 0xff;
+        val &= cpu->v7m_priority_mask;
         if (val != 0 && (val < env->v7m.basepri || env->v7m.basepri == 0))
             env->v7m.basepri = val;
         break;
-- 
2.1.4