hw/vfio: generate new file fd for pseudo device and rebind existing descriptors

Normally the vfio pseudo device file descriptor lives for the life of the VM.
However, when the kvm VM file descriptor changes, a new file descriptor
for the pseudo device needs to be generated against the new kvm VM descriptor.
Other existing vfio descriptors needs to be reattached to the new pseudo device
descriptor. This change performs the above steps.

Tested-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Ani Sinha <anisinha@redhat.com>
Link: https://lore.kernel.org/r/20260227072445.406907-1-anisinha@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/vfio/helpers.c

@@ -116,6 +116,88 @@ bool vfio_get_info_dma_avail(struct vfio_iommu_type1_info *info,
  * we'll re-use it should another vfio device be attached before then.
  */
 int vfio_kvm_device_fd = -1;
+
+/*
+ * Confidential virtual machines:
+ * During reset of confidential vms, the kvm vm file descriptor changes.
+ * In this case, the old vfio kvm file descriptor is
+ * closed and a new descriptor is created against the new kvm vm file
+ * descriptor.
+ */
+
+typedef struct VFIODeviceFd {
+    int fd;
+    QLIST_ENTRY(VFIODeviceFd) node;
+} VFIODeviceFd;
+
+static QLIST_HEAD(, VFIODeviceFd) vfio_device_fds =
+    QLIST_HEAD_INITIALIZER(vfio_device_fds);
+
+static void vfio_device_fd_list_add(int fd)
+{
+    VFIODeviceFd *file_fd;
+    file_fd = g_malloc0(sizeof(*file_fd));
+    file_fd->fd = fd;
+    QLIST_INSERT_HEAD(&vfio_device_fds, file_fd, node);
+}
+
+static void vfio_device_fd_list_remove(int fd)
+{
+    VFIODeviceFd *file_fd, *next;
+
+    QLIST_FOREACH_SAFE(file_fd, &vfio_device_fds, node, next) {
+        if (file_fd->fd == fd) {
+            QLIST_REMOVE(file_fd, node);
+            g_free(file_fd);
+            break;
+        }
+    }
+}
+
+static int vfio_device_fd_rebind(NotifierWithReturn *notifier, void *data,
+                                  Error **errp)
+{
+    VFIODeviceFd *file_fd;
+    struct kvm_device_attr attr = {
+        .group = KVM_DEV_VFIO_FILE,
+        .attr = KVM_DEV_VFIO_FILE_ADD,
+    };
+    struct kvm_create_device cd = {
+        .type = KVM_DEV_TYPE_VFIO,
+    };
+
+    /* we are not interested in pre vmfd change notification */
+    if (((VmfdChangeNotifier *)data)->pre) {
+        return 0;
+    }
+
+    if (kvm_vm_ioctl(kvm_state, KVM_CREATE_DEVICE, &cd)) {
+        error_setg_errno(errp, errno, "Failed to create KVM VFIO device");
+        return -errno;
+    }
+
+    if (vfio_kvm_device_fd != -1) {
+        close(vfio_kvm_device_fd);
+    }
+
+    vfio_kvm_device_fd = cd.fd;
+
+    QLIST_FOREACH(file_fd, &vfio_device_fds, node) {
+        attr.addr = (uint64_t)(unsigned long)&file_fd->fd;
+        if (ioctl(vfio_kvm_device_fd, KVM_SET_DEVICE_ATTR, &attr)) {
+            error_setg_errno(errp, errno,
+                             "Failed to add fd %d to KVM VFIO device",
+                             file_fd->fd);
+            return -errno;
+        }
+    }
+    return 0;
+}
+
+static struct NotifierWithReturn vfio_vmfd_change_notifier = {
+    .notify = vfio_device_fd_rebind,
+};
+
 #endif
 
 void vfio_kvm_device_close(void)
@@ -153,6 +235,11 @@ int vfio_kvm_device_add_fd(int fd, Error **errp)
         }
 
         vfio_kvm_device_fd = cd.fd;
+        /*
+         * If the vm file descriptor changes, add a notifier so that we can
+         * re-create the vfio_kvm_device_fd.
+         */
+        kvm_vmfd_add_change_notifier(&vfio_vmfd_change_notifier);
     }
 
     if (ioctl(vfio_kvm_device_fd, KVM_SET_DEVICE_ATTR, &attr)) {
@@ -160,6 +247,8 @@ int vfio_kvm_device_add_fd(int fd, Error **errp)
                          fd);
         return -errno;
     }
+
+    vfio_device_fd_list_add(fd);
 #endif
     return 0;
 }
@@ -183,6 +272,8 @@ int vfio_kvm_device_del_fd(int fd, Error **errp)
                          "Failed to remove fd %d from KVM VFIO device", fd);
         return -errno;
     }
+
+    vfio_device_fd_list_remove(fd);
 #endif
     return 0;
 }