arm_gic: Mask the un-supported priority bits

The GICv2 allows the implementation to implement a variable number
of priority bits; unimplemented bits in the priority registers
are read as zeros, writes ignored. We were previously always
implementing a full 8 bits of priority, which is allowed but not
what the real hardware typically does (which is usually to have
4 or 5 bits of priority).

Add a new device property to allow the number of implemented
property bits to be specified.

Signed-off-by: Sai Pavan Boddu <sai.pavan.boddu@xilinx.com>
Message-id: 1582537164-764-2-git-send-email-sai.pavan.boddu@xilinx.com
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: improved commit message]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/intc/arm_gic.c

@@ -641,6 +641,23 @@ uint32_t gic_acknowledge_irq(GICState *s, int cpu, MemTxAttrs attrs)
     return ret;
 }
 
+static uint32_t gic_fullprio_mask(GICState *s, int cpu)
+{
+    /*
+     * Return a mask word which clears the unimplemented priority
+     * bits from a priority value for an interrupt. (Not to be
+     * confused with the group priority, whose mask depends on BPR.)
+     */
+    int priBits;
+
+    if (gic_is_vcpu(cpu)) {
+        priBits = GIC_VIRT_MAX_GROUP_PRIO_BITS;
+    } else {
+        priBits = s->n_prio_bits;
+    }
+    return ~0U << (8 - priBits);
+}
+
 void gic_dist_set_priority(GICState *s, int cpu, int irq, uint8_t val,
                       MemTxAttrs attrs)
 {
@@ -651,6 +668,8 @@ void gic_dist_set_priority(GICState *s, int cpu, int irq, uint8_t val,
         val = 0x80 | (val >> 1); /* Non-secure view */
     }
 
+    val &= gic_fullprio_mask(s, cpu);
+
     if (irq < GIC_INTERNAL) {
         s->priority1[irq][cpu] = val;
     } else {
@@ -669,7 +688,7 @@ static uint32_t gic_dist_get_priority(GICState *s, int cpu, int irq,
         }
         prio = (prio << 1) & 0xff; /* Non-secure view */
     }
-    return prio;
+    return prio & gic_fullprio_mask(s, cpu);
 }
 
 static void gic_set_priority_mask(GICState *s, int cpu, uint8_t pmask,
@@ -684,7 +703,7 @@ static void gic_set_priority_mask(GICState *s, int cpu, uint8_t pmask,
             return;
         }
     }
-    s->priority_mask[cpu] = pmask;
+    s->priority_mask[cpu] = pmask & gic_fullprio_mask(s, cpu);
 }
 
 static uint32_t gic_get_priority_mask(GICState *s, int cpu, MemTxAttrs attrs)
@@ -2055,6 +2074,16 @@ static void arm_gic_realize(DeviceState *dev, Error **errp)
         return;
     }
 
+    if (s->n_prio_bits > GIC_MAX_PRIORITY_BITS ||
+       (s->virt_extn ? s->n_prio_bits < GIC_VIRT_MAX_GROUP_PRIO_BITS :
+        s->n_prio_bits < GIC_MIN_PRIORITY_BITS)) {
+        error_setg(errp, "num-priority-bits cannot be greater than %d"
+                   " or less than %d", GIC_MAX_PRIORITY_BITS,
+                   s->virt_extn ? GIC_VIRT_MAX_GROUP_PRIO_BITS :
+                   GIC_MIN_PRIORITY_BITS);
+        return;
+    }
+
     /* This creates distributor, main CPU interface (s->cpuiomem[0]) and if
      * enabled, virtualization extensions related interfaces (main virtual
      * interface (s->vifaceiomem[0]) and virtual CPU interface).

hw/intc/arm_gic_common.c

@@ -357,6 +357,7 @@ static Property arm_gic_common_properties[] = {
     DEFINE_PROP_BOOL("has-security-extensions", GICState, security_extn, 0),
     /* True if the GIC should implement the virtualization extensions */
     DEFINE_PROP_BOOL("has-virtualization-extensions", GICState, virt_extn, 0),
+    DEFINE_PROP_UINT32("num-priority-bits", GICState, n_prio_bits, 8),
     DEFINE_PROP_END_OF_LIST(),
 };
 

include/hw/intc/arm_gic.h

@@ -68,6 +68,8 @@
 
 /* Number of SGI target-list bits */
 #define GIC_TARGETLIST_BITS 8
+#define GIC_MAX_PRIORITY_BITS 8
+#define GIC_MIN_PRIORITY_BITS 4
 
 #define TYPE_ARM_GIC "arm_gic"
 #define ARM_GIC(obj) \

include/hw/intc/arm_gic_common.h

@@ -96,6 +96,7 @@ typedef struct GICState {
     uint16_t priority_mask[GIC_NCPU_VCPU];
     uint16_t running_priority[GIC_NCPU_VCPU];
     uint16_t current_pending[GIC_NCPU_VCPU];
+    uint32_t n_prio_bits;
 
     /* If we present the GICv2 without security extensions to a guest,
      * the guest can configure the GICC_CTLR to configure group 1 binary point