[PATCH 2/5] hw/intc/arm_gicv3_kvm.c: Stop using GIC_MIN_BPR constant

[Peter Maydell <peter.maydell@linaro.org>]

The GIC_MIN_BPR constant defines the minimum BPR value that the TCG
emulated GICv3 supports.  We're currently using this also as the
value we reset the KVM GICv3 ICC_BPR registers to, but this is only
right by accident.

We want to make the emulated GICv3 use a configurable number of
priority bits, which means that GIC_MIN_BPR will no longer be a
constant.  Replace the uses in the KVM reset code with literal 0,
plus a constant explaining why this is reasonable.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 hw/intc/arm_gicv3_kvm.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/hw/intc/arm_gicv3_kvm.c b/hw/intc/arm_gicv3_kvm.c
index 2922c516e56..3ca643ecba4 100644
--- a/hw/intc/arm_gicv3_kvm.c
+++ b/hw/intc/arm_gicv3_kvm.c
@@ -673,9 +673,19 @@ static void arm_gicv3_icc_reset(CPUARMState *env, const ARMCPRegInfo *ri)
     s = c->gic;
 
     c->icc_pmr_el1 = 0;
-    c->icc_bpr[GICV3_G0] = GIC_MIN_BPR;
-    c->icc_bpr[GICV3_G1] = GIC_MIN_BPR;
-    c->icc_bpr[GICV3_G1NS] = GIC_MIN_BPR;
+    /*
+     * Architecturally the reset value of the ICC_BPR registers
+     * is UNKNOWN. We set them all to 0 here; when the kernel
+     * uses these values to program the ICH_VMCR_EL2 fields that
+     * determine the guest-visible ICC_BPR register values, the
+     * hardware's "writing a value less than the minimum sets
+     * the field to the minimum value" behaviour will result in
+     * them effectively resetting to the correct minimum value
+     * for the host GIC.
+     */
+    c->icc_bpr[GICV3_G0] = 0;
+    c->icc_bpr[GICV3_G1] = 0;
+    c->icc_bpr[GICV3_G1NS] = 0;
 
     c->icc_sre_el1 = 0x7;
     memset(c->icc_apr, 0, sizeof(c->icc_apr));
-- 
2.25.1