[PATCH 1/2] KVM: introduce a 'release' method for KVM devices

["Cédric Le Goater" <clg@kaod.org>]

When a P9 sPAPR VM boots, the CAS negotiation process determines which
interrupt mode to use (XICS legacy or XIVE native) and invokes a
machine reset to activate the chosen mode.

To be able to switch from one interrupt mode to another, we introduce
the capability to release a KVM device without destroying the VM. The
KVM device interface is extended with a new 'release' method which is
called when the file descriptor of the device is closed.

Once 'release' is called, the 'destroy' method will not be called
anymore as the device is removed from the device list of the VM.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
---
 include/linux/kvm_host.h |  9 +++++++++
 virt/kvm/kvm_main.c      | 13 +++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 831d963451d8..722692e2f745 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1240,6 +1240,15 @@ struct kvm_device_ops {
 	 */
 	void (*destroy)(struct kvm_device *dev);
 
+	/*
+	 * Release is an alternative method to free the device. It is
+	 * called when the device file descriptor is closed. Once
+	 * release is called, the destroy method will not be called
+	 * anymore as the device is removed from the device list of
+	 * the VM. kvm->lock is held.
+	 */
+	void (*release)(struct kvm_device *dev);
+
 	int (*set_attr)(struct kvm_device *dev, struct kvm_device_attr *attr);
 	int (*get_attr)(struct kvm_device *dev, struct kvm_device_attr *attr);
 	int (*has_attr)(struct kvm_device *dev, struct kvm_device_attr *attr);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index ea2018ae1cd7..ea2619d5ca98 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2938,6 +2938,19 @@ static int kvm_device_release(struct inode *inode, struct file *filp)
 	struct kvm_device *dev = filp->private_data;
 	struct kvm *kvm = dev->kvm;
 
+	if (!dev)
+		return -ENODEV;
+
+	if (dev->kvm != kvm)
+		return -EPERM;
+
+	if (dev->ops->release) {
+		mutex_lock(&kvm->lock);
+		list_del(&dev->vm_node);
+		dev->ops->release(dev);
+		mutex_unlock(&kvm->lock);
+	}
+
 	kvm_put_kvm(kvm);
 	return 0;
 }
-- 
2.20.1