hw/intc/arm_gicv3_cpuif: Fix priority masking for NS BPR1

When we calculate the mask to use to get the group priority from
an interrupt priority, the way that NS BPR1 is handled differs
from how BPR0 and S BPR1 work -- a BPR1 value of 1 means
the group priority is in bits [7:1], whereas for BPR0 and S BPR1
this is indicated by a 0 BPR value.

Subtract 1 from the BPR value before creating the mask if
we're using the NS BPR value, for both hardware and virtual
interrupts, as the GICv3 pseudocode does, and fix the comments
accordingly.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 1493226792-3237-4-git-send-email-peter.maydell@linaro.org

hw/intc/arm_gicv3_cpuif.c

@@ -216,18 +216,35 @@ static uint32_t icv_gprio_mask(GICv3CPUState *cs, int group)
 {
     /* Return a mask word which clears the subpriority bits from
      * a priority value for a virtual interrupt in the specified group.
-     * This depends on the VBPR value:
+     * This depends on the VBPR value.
+     * If using VBPR0 then:
      *  a BPR of 0 means the group priority bits are [7:1];
      *  a BPR of 1 means they are [7:2], and so on down to
      *  a BPR of 7 meaning no group priority bits at all.
+     * If using VBPR1 then:
+     *  a BPR of 0 is impossible (the minimum value is 1)
+     *  a BPR of 1 means the group priority bits are [7:1];
+     *  a BPR of 2 means they are [7:2], and so on down to
+     *  a BPR of 7 meaning the group priority is [7].
+     *
      * Which BPR to use depends on the group of the interrupt and
      * the current ICH_VMCR_EL2.VCBPR settings.
+     *
+     * This corresponds to the VGroupBits() pseudocode.
      */
+    int bpr;
+
     if (group == GICV3_G1NS && cs->ich_vmcr_el2 & ICH_VMCR_EL2_VCBPR) {
         group = GICV3_G0;
     }
 
-    return ~0U << (read_vbpr(cs, group) + 1);
+    bpr = read_vbpr(cs, group);
+    if (group == GICV3_G1NS) {
+        assert(bpr > 0);
+        bpr--;
+    }
+
+    return ~0U << (bpr + 1);
 }
 
 static bool icv_hppi_can_preempt(GICv3CPUState *cs, uint64_t lr)
@@ -674,20 +691,37 @@ static uint32_t icc_gprio_mask(GICv3CPUState *cs, int group)
 {
     /* Return a mask word which clears the subpriority bits from
      * a priority value for an interrupt in the specified group.
-     * This depends on the BPR value:
+     * This depends on the BPR value. For CBPR0 (S or NS):
      *  a BPR of 0 means the group priority bits are [7:1];
      *  a BPR of 1 means they are [7:2], and so on down to
      *  a BPR of 7 meaning no group priority bits at all.
+     * For CBPR1 NS:
+     *  a BPR of 0 is impossible (the minimum value is 1)
+     *  a BPR of 1 means the group priority bits are [7:1];
+     *  a BPR of 2 means they are [7:2], and so on down to
+     *  a BPR of 7 meaning the group priority is [7].
+     *
      * Which BPR to use depends on the group of the interrupt and
      * the current ICC_CTLR.CBPR settings.
+     *
+     * This corresponds to the GroupBits() pseudocode.
      */
+    int bpr;
+
     if ((group == GICV3_G1 && cs->icc_ctlr_el1[GICV3_S] & ICC_CTLR_EL1_CBPR) ||
         (group == GICV3_G1NS &&
          cs->icc_ctlr_el1[GICV3_NS] & ICC_CTLR_EL1_CBPR)) {
         group = GICV3_G0;
     }
 
-    return ~0U << ((cs->icc_bpr[group] & 7) + 1);
+    bpr = cs->icc_bpr[group] & 7;
+
+    if (group == GICV3_G1NS) {
+        assert(bpr > 0);
+        bpr--;
+    }
+
+    return ~0U << (bpr + 1);
 }
 
 static bool icc_no_enabled_hppi(GICv3CPUState *cs)