[RFC PATCH v1 36/42] KVM: plane: handle KVM_CHECK_EXTENSION on the plane fd

A plane file descriptor did not implement KVM_CHECK_EXTENSION and failed
with -ENOTTY, but userspace needs to be able to query capabilities on it.
Forward the query to the plane's parent VM, except for KVM_CAP_PLANES
which returns 0 because a plane cannot host planes of its own.
---
 virt/kvm/kvm_main.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 3a1a09f26340..6e4f3f3e6881 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4946,12 +4946,21 @@ static long kvm_vcpu_compat_ioctl(struct file *filp,
 }
 #endif
 
+static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg);
+
 static long __kvm_plane_ioctl(struct kvm_plane *plane, unsigned int ioctl, unsigned long arg)
 {
 	void __user *argp = (void __user *)arg;
 	long r;
 
 	switch (ioctl) {
+	case KVM_CHECK_EXTENSION:
+		/* A plane cannot host planes of its own. */
+		if (arg == KVM_CAP_PLANES)
+			r = 0;
+		else
+			r = kvm_vm_ioctl_check_extension_generic(plane->kvm, arg);
+		break;
 	case KVM_CREATE_VCPU:
 		r = kvm_plane_ioctl_create_vcpu(plane, arg);
 		break;
-- 
2.55.0