[RFC 2/2] hw/arm/virt: kvm: allow gicv3 by default if host does not support v2

[Eric Auger <eric.auger@redhat.com>]

At the moment if the end-user does not specify the gic-version along
with KVM acceleration, v2 is set by default. However most of the
systems now have GICv3 and sometimes they do not support GICv2
compatibility. In that case we end up with this error:

qemu-system-aarch64: PMU: KVM_SET_DEVICE_ATTR: Invalid argument
qemu-system-aarch64: failed to set irq for PMU
and qemu aborts.

This patch keeps the default v2 selection in all cases except
in the KVM accelerated mode when the host does not support v2.
This case did not work anyway so we do not break any compatibility.
Now we get v3 selected in such a case. Also if the end-user explicitly
sets v2 whereas this latter is not supported, we also are
informed that v2 is not selected by thos host instead of getting the
above PMU related message.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 hw/arm/virt.c         | 30 ++++++++++++++++++++++++++++++
 include/hw/arm/virt.h |  1 +
 2 files changed, 31 insertions(+)

diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index a196bbf0d5..b37b0c40c1 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -1554,6 +1554,33 @@ static void machvirt_init(MachineState *machine)
                 }
             }
         }
+    } else if (kvm_enabled()) {
+        int probe_bitmap = kvm_arm_vgic_probe();
+
+        if (!probe_bitmap) {
+            error_report(
+                "Unable to determine GIC version supported by host");
+            exit(1);
+        }
+        if (!vms->gic_version_user_selected) {
+            /*
+             * by default v2 is supposed to be chosen: check it is
+             * supported by the host. Otherwise take v3.
+             */
+            if (probe_bitmap & KVM_ARM_VGIC_V2) {
+                vms->gic_version = 2;
+            } else if (probe_bitmap & KVM_ARM_VGIC_V3) {
+                vms->gic_version = 3;
+            }
+        } else { /* user explicitly set the version to 2 or 3 */
+            if (vms->gic_version == 2 && !(probe_bitmap & KVM_ARM_VGIC_V2)) {
+                error_report("GICv2 is not supported by the host");
+                exit(1);
+            } else if (vms->gic_version == 3 && !(probe_bitmap & KVM_ARM_VGIC_V3)) {
+                error_report("GICv3 is not supported by the host");
+                exit(1);
+            }
+        }
     }
 
     if (!cpu_type_valid(machine->cpu_type)) {
@@ -1840,6 +1867,7 @@ static void virt_set_gic_version(Object *obj, const char *value, Error **errp)
 {
     VirtMachineState *vms = VIRT_MACHINE(obj);
 
+    vms->gic_version_user_selected = true;
     if (!strcmp(value, "3")) {
         vms->gic_version = 3;
     } else if (!strcmp(value, "2")) {
@@ -1851,6 +1879,7 @@ static void virt_set_gic_version(Object *obj, const char *value, Error **errp)
     } else {
         error_setg(errp, "Invalid gic-version value");
         error_append_hint(errp, "Valid values are 3, 2, host, max.\n");
+        vms->gic_version_user_selected = false;
     }
 }
 
@@ -2103,6 +2132,7 @@ static void virt_instance_init(Object *obj)
                                     NULL);
     /* Default GIC type is v2 */
     vms->gic_version = 2;
+    vms->gic_version_user_selected = false;
     object_property_add_str(obj, "gic-version", virt_get_gic_version,
                         virt_set_gic_version, NULL);
     object_property_set_description(obj, "gic-version",
diff --git a/include/hw/arm/virt.h b/include/hw/arm/virt.h
index 71508bf40c..e363cde452 100644
--- a/include/hw/arm/virt.h
+++ b/include/hw/arm/virt.h
@@ -124,6 +124,7 @@ typedef struct {
     bool its;
     bool virt;
     int32_t gic_version;
+    bool gic_version_user_selected;
     VirtIOMMUType iommu;
     struct arm_boot_info bootinfo;
     MemMapEntry *memmap;
-- 
2.20.1