[RFC PATCH v3 09/13] KVM: guest_memfd: Expose device provider private state

["Aneesh Kumar K.V (Arm)" <aneesh.kumar@kernel.org>]

Expose a callback to query whether private ranges remain. Initialize it
before attaching the device provider so the vDEVICE can coordinate
protected cleanup. Keep the summary inode-owned and valid until release;
providers may access the VM only during attach and core callbacks.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: kvm@vger.kernel.org
Cc: patches@lists.linux.dev
Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
---
 include/linux/guest_memfd.h |  7 +++++++
 virt/kvm/guest_memfd.c      | 10 ++++++++++
 2 files changed, 17 insertions(+)

diff --git a/include/linux/guest_memfd.h b/include/linux/guest_memfd.h
index 8a93004aa9e7..5e5725398ab0 100644
--- a/include/linux/guest_memfd.h
+++ b/include/linux/guest_memfd.h
@@ -24,12 +24,19 @@ struct guest_memfd_device_binding {
  * @flags: GUEST_MEMFD_FLAG_* creation flags
  * @binding: Mapping owner and device ID set by attach(), with the underlying
  *           resources pinned until release()
+ * @has_private: Check whether any committed private ranges remain
+ *
+ * The VM is valid during attach() and core-initiated callbacks. Providers
+ * must not use this pointer for asynchronous VM access.
+ * release() may run after the VM reference has been dropped. Inode-owned
+ * metadata, including the private-range summary, remains valid until then.
  */
 struct guest_memfd_device {
 	struct kvm *kvm;
 	u64 size;
 	u64 flags;
 	struct guest_memfd_device_binding binding;
+	bool (*has_private)(struct guest_memfd_device *gmem);
 };
 
 /**
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index af1e4798ace1..1322576350b8 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -85,6 +85,7 @@ static bool gmem_provider_is_device(struct gmem_inode *gi)
 
 static void kvm_gmem_device_update_attributes(struct inode *inode);
 static int kvm_gmem_device_make_private(struct inode *inode, pgoff_t start, pgoff_t end);
+static bool gmem_device_has_private(struct guest_memfd_device *gdev);
 
 struct guest_memfd_device *kvm_gmem_get_device(struct inode *inode)
 {
@@ -176,6 +177,7 @@ static int kvm_gmem_attach_resource(struct kvm *kvm, struct file *file,
 			gi->device.kvm = kvm;
 			gi->device.size = i_size_read(inode);
 			gi->device.flags = gi->flags;
+			gi->device.has_private = gmem_device_has_private;
 		}
 		provider = ops->attach(inode, resource_file);
 	}
@@ -1510,6 +1512,14 @@ int __weak kvm_arch_gmem_device_map(struct kvm *kvm,
 	return -EOPNOTSUPP;
 }
 
+/* The attribute tree owns mapping state; this summary is read under provider locks. */
+static bool gmem_device_has_private(struct guest_memfd_device *gdev)
+{
+	struct gmem_inode *gi = container_of(gdev, struct gmem_inode, device);
+
+	return READ_ONCE(gi->device_has_private);
+}
+
 static void kvm_gmem_device_update_attributes(struct inode *inode)
 {
 	struct gmem_inode *gi = GMEM_I(inode);
-- 
2.43.0