gpu: nvgpu: domains as files for access control

Create device nodes for user-created scheduling domains. This helps
leverage filesystem based access control: domains can be chosen to be
available for a limited set of users on a system.

The device nodes are dynamic: they can be removed while the driver is
running normally. This is a bit different from the nodes that exist
until the driver is unloaded, so the devno/domain mapping is stored in a
separate list. The usual container_of pattern would suffer from an
unavoidable race condition if a domain file was opened while the same
domain would get removed.

As usual, domain refcounting prevents a domain from being removed. Now
the open device files hold refs and thus any open domain files prevent a
domain from getting removed, in addition to the userspace-invisible ref
that is taken when a TSG is bound to a domain.

While at it, make the query ioctl guarded by the sched domain mutex, as
domains might technically get added or removed during the querying code.

Jira NVGPU-6788

Change-Id: Ief2a09a442c4e70f1f2be8a32359341071d74659
Signed-off-by: Konsta Hölttä <kholtta@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/c/linux-nvgpu/+/2651164
Reviewed-by: Alex Waterman <alexw@nvidia.com>
GVS: Gerrit_Virtual_Submit

include/uapi/linux/nvgpu-nvs.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, NVIDIA CORPORATION.  All rights reserved.
+ * Copyright (c) 2021-2022, NVIDIA CORPORATION.  All rights reserved.
  *
  * This program is free software; you can redistribute it and/or modify it
  * under the terms and conditions of the GNU General Public License,
@@ -87,6 +87,14 @@ struct nvgpu_nvs_ioctl_create_domain {
 	__u64 reserved1;
 };
 
+/**
+ * NVGPU_NVS_IOCTL_REMOVE_DOMAIN
+ *
+ * Remove a domain that has been previously created.
+ *
+ * The domain must be empty; it must have no TSGs bound to it. The domain's
+ * device node must not be open by anyone.
+ */
 struct nvgpu_nvs_ioctl_remove_domain {
 	/*
 	 * In: a domain_id to remove.