Host kernel within [4.18, 5.3] report an erroneous KVM_MAX_VCPUS=512
for ARM. The actual capability to instantiate more than 256 vcpus
was fixed in 5.4 with the upgrade of the KVM_IRQ_LINE ABI to support
vcpu id encoded on 12 bits instead of 8 and a redistributor consuming
a single KVM IO device instead of 2.

So let's check this capability when attempting to use more than 256
vcpus.

Signed-off-by: Eric Auger <eric.au...@redhat.com>
---
 hw/arm/virt.c        |  4 ++++
 target/arm/kvm.c     | 21 +++++++++++++++++++++
 target/arm/kvm_arm.h | 15 +++++++++++++++
 3 files changed, 40 insertions(+)

diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index 0d1629ccb3..465e3140f7 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -1575,6 +1575,10 @@ static void machvirt_init(MachineState *machine)
         virt_max_cpus = GIC_NCPU;
     }
 
+    if (kvm_arm_irq_line_layout_mismatch(MACHINE(vms), max_cpus)) {
+        exit(1);
+    }
+
     if (max_cpus > virt_max_cpus) {
         error_report("Number of SMP CPUs requested (%d) exceeds max CPUs "
                      "supported by machine 'mach-virt' (%d)",
diff --git a/target/arm/kvm.c b/target/arm/kvm.c
index 6cdfa2204f..b601e2f35a 100644
--- a/target/arm/kvm.c
+++ b/target/arm/kvm.c
@@ -171,6 +171,27 @@ bool kvm_arm_pmu_supported(CPUState *cpu)
     return kvm_check_extension(s, KVM_CAP_ARM_PMU_V3);
 }
 
+bool kvm_arm_irq_line_layout_mismatch(MachineState *ms, int vcpus)
+{
+    KVMState *s;
+    bool ret;
+
+    if (!kvm_enabled()) {
+        return false;
+    }
+
+    s = KVM_STATE(ms->accelerator);
+
+    ret = vcpus > 256 &&
+          !kvm_check_extension(s, KVM_CAP_ARM_IRQ_LINE_LAYOUT_2);
+
+    if (ret) {
+        error_report("Using more than 256 vcpus requires a host kernel "
+                     "with KVM_CAP_ARM_IRQ_LINE_LAYOUT_2");
+    }
+    return ret;
+}
+
 int kvm_arm_get_max_vm_ipa_size(MachineState *ms)
 {
     KVMState *s = KVM_STATE(ms->accelerator);
diff --git a/target/arm/kvm_arm.h b/target/arm/kvm_arm.h
index b4e19457a0..d893d950d8 100644
--- a/target/arm/kvm_arm.h
+++ b/target/arm/kvm_arm.h
@@ -233,6 +233,16 @@ bool kvm_arm_pmu_supported(CPUState *cs);
  */
 int kvm_arm_get_max_vm_ipa_size(MachineState *ms);
 
+/**
+ * kvm_arm_irq_line_layout_mismatch - Returns whether the number of vcpus
+ * exceeds the limit imposed by the legacy KVM_IRQ_LINE ARM layout
+ * (without the vcpu2_index field).
+ *
+ * @ms: Machine state handle
+ * @vcpus: number of vcpus
+ */
+bool kvm_arm_irq_line_layout_mismatch(MachineState *ms, int vcpus);
+
 /**
  * kvm_arm_sync_mpstate_to_kvm
  * @cpu: ARMCPU
@@ -281,6 +291,11 @@ static inline int kvm_arm_get_max_vm_ipa_size(MachineState 
*ms)
     return -ENOENT;
 }
 
+static inline bool kvm_arm_irq_line_layout_mismatch(MachineState *ms, int 
vcpus)
+{
+    return false;
+}
+
 static inline int kvm_arm_vgic_probe(void)
 {
     return 0;
-- 
2.20.1


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