kvm: vfio: fix unregister kvm_device_ops of vfio

After commit 80ce1639 (KVM: VFIO: register kvm_device_ops dynamically),
kvm_device_ops of vfio can be registered dynamically. Commit 3c3c29fd
(kvm-vfio: do not use module_init) move the dynamic register invoked by
kvm_init in order to fix broke unloading of the kvm module. However,
kvm_device_ops of vfio is unregistered after rmmod kvm-intel module
which lead to device type collision detection warning after kvm-intel
module reinsmod.

    WARNING: CPU: 1 PID: 10358 at /root/cathy/kvm/arch/x86/kvm/../../../virt/kvm/kvm_main.c:3289 kvm_init+0x234/0x282 [kvm]()
    Modules linked in: kvm_intel(O+) kvm(O) nfsv3 nfs_acl auth_rpcgss oid_registry nfsv4 dns_resolver nfs fscache lockd sunrpc pci_stub bridge stp llc autofs4 8021q cpufreq_ondemand ipv6 joydev microcode pcspkr igb i2c_algo_bit ehci_pci ehci_hcd e1000e i2c_i801 ixgbe ptp pps_core hwmon mdio tpm_tis tpm ipmi_si ipmi_msghandler acpi_cpufreq isci libsas scsi_transport_sas button dm_mirror dm_region_hash dm_log dm_mod [last unloaded: kvm_intel]
    CPU: 1 PID: 10358 Comm: insmod Tainted: G        W  O   3.17.0-rc1 #2
    Hardware name: Intel Corporation S2600CP/S2600CP, BIOS RMLSDP.86I.00.29.D696.1311111329 11/11/2013
     0000000000000cd9 ffff880ff08cfd18 ffffffff814a61d9 0000000000000cd9
     0000000000000000 ffff880ff08cfd58 ffffffff810417b7 ffff880ff08cfd48
     ffffffffa045bcac ffffffffa049c420 0000000000000040 00000000000000ff
    Call Trace:
     [<ffffffff814a61d9>] dump_stack+0x49/0x60
     [<ffffffff810417b7>] warn_slowpath_common+0x7c/0x96
     [<ffffffffa045bcac>] ? kvm_init+0x234/0x282 [kvm]
     [<ffffffff810417e6>] warn_slowpath_null+0x15/0x17
     [<ffffffffa045bcac>] kvm_init+0x234/0x282 [kvm]
     [<ffffffffa016e995>] vmx_init+0x1bf/0x42a [kvm_intel]
     [<ffffffffa016e7d6>] ? vmx_check_processor_compat+0x64/0x64 [kvm_intel]
     [<ffffffff810002ab>] do_one_initcall+0xe3/0x170
     [<ffffffff811168a9>] ? __vunmap+0xad/0xb8
     [<ffffffff8109c58f>] do_init_module+0x2b/0x174
     [<ffffffff8109d414>] load_module+0x43e/0x569
     [<ffffffff8109c6d8>] ? do_init_module+0x174/0x174
     [<ffffffff8109c75a>] ? copy_module_from_user+0x39/0x82
     [<ffffffff8109b7dd>] ? module_sect_show+0x20/0x20
     [<ffffffff8109d65f>] SyS_init_module+0x54/0x81
     [<ffffffff814a9a12>] system_call_fastpath+0x16/0x1b
    ---[ end trace 0626f4a3ddea56f3 ]---

The bug can be reproduced by:

    rmmod kvm_intel.ko
    insmod kvm_intel.ko

without rmmod/insmod kvm.ko
This patch fixes the bug by unregistering kvm_device_ops of vfio when the
kvm-intel module is removed.
Reported-by: NLiu Rongrong <rongrongx.liu@intel.com>
Fixes: 3c3c29fdSigned-off-by: NWanpeng Li <wanpeng.li@linux.intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

include/linux/kvm_host.h

@@ -1080,6 +1080,7 @@ void kvm_device_get(struct kvm_device *dev);
 void kvm_device_put(struct kvm_device *dev);
 struct kvm_device *kvm_device_from_filp(struct file *filp);
 int kvm_register_device_ops(struct kvm_device_ops *ops, u32 type);
+void kvm_unregister_device_ops(u32 type);
 
 extern struct kvm_device_ops kvm_mpic_ops;
 extern struct kvm_device_ops kvm_xics_ops;

virt/kvm/kvm_main.c

@@ -2354,6 +2354,12 @@ int kvm_register_device_ops(struct kvm_device_ops *ops, u32 type)
 	return 0;
 }
 
+void kvm_unregister_device_ops(u32 type)
+{
+	if (kvm_device_ops_table[type] != NULL)
+		kvm_device_ops_table[type] = NULL;
+}
+
 static int kvm_ioctl_create_device(struct kvm *kvm,
 				   struct kvm_create_device *cd)
 {
@@ -3328,5 +3334,6 @@ void kvm_exit(void)
 	kvm_arch_exit();
 	kvm_irqfd_exit();
 	free_cpumask_var(cpus_hardware_enabled);
+	kvm_vfio_ops_exit();
 }
 EXPORT_SYMBOL_GPL(kvm_exit);

virt/kvm/vfio.c

@@ -283,3 +283,8 @@ int kvm_vfio_ops_init(void)
 {
 	return kvm_register_device_ops(&kvm_vfio_ops, KVM_DEV_TYPE_VFIO);
 }
+
+void kvm_vfio_ops_exit(void)
+{
+	kvm_unregister_device_ops(KVM_DEV_TYPE_VFIO);
+}

virt/kvm/vfio.h

@@ -3,11 +3,15 @@
 
 #ifdef CONFIG_KVM_VFIO
 int kvm_vfio_ops_init(void);
+void kvm_vfio_ops_exit(void);
 #else
 static inline int kvm_vfio_ops_init(void)
 {
 	return 0;
 }
+static inline void kvm_vfio_ops_exit(void)
+{
+}
 #endif
 
 #endif