From: Eric Auger <eric.auger@redhat.com>

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 within any ARM kvm accelerated machine.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Message-id: 20191003154640.22451-4-eric.auger@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 target/arm/kvm.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/target/arm/kvm.c b/target/arm/kvm.c
index b10581fa066..b473c63edb1 100644
--- a/target/arm/kvm.c
+++ b/target/arm/kvm.c
@@ -182,6 +182,7 @@ int kvm_arm_get_max_vm_ipa_size(MachineState *ms)
 
 int kvm_arch_init(MachineState *ms, KVMState *s)
 {
+    int ret = 0;
     /* For ARM interrupt delivery is always asynchronous,
      * whether we are using an in-kernel VGIC or not.
      */
@@ -195,7 +196,14 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
 
     cap_has_mp_state = kvm_check_extension(s, KVM_CAP_MP_STATE);
 
-    return 0;
+    if (ms->smp.cpus > 256 &&
+        !kvm_check_extension(s, KVM_CAP_ARM_IRQ_LINE_LAYOUT_2)) {
+        error_report("Using more than 256 vcpus requires a host kernel "
+                     "with KVM_CAP_ARM_IRQ_LINE_LAYOUT_2");
+        ret = -EINVAL;
+    }
+
+    return ret;
 }
 
 unsigned long kvm_arch_vcpu_id(CPUState *cpu)
-- 
2.20.1