drm/i915: Introduce the i915_user_extension_method

An idea for extending uABI inspired by Vulkan's extension chains.
Instead of expanding the data struct for each ioctl every time we need
to add a new feature, define an extension chain instead. As we add
optional interfaces to control the ioctl, we define a new extension
struct that can be linked into the ioctl data only when required by the
user. The key advantage being able to ignore large control structs for
optional interfaces/extensions, while being able to process them in a
consistent manner.

In comparison to other extensible ioctls, the key difference is the
use of a linked chain of extension structs vs an array of tagged
pointers. For example,

struct drm_amdgpu_cs_chunk {
        __u32           chunk_id;
        __u32           length_dw;
        __u64           chunk_data;
};

struct drm_amdgpu_cs_in {
        __u32           ctx_id;
        __u32           bo_list_handle;
        __u32           num_chunks;
        __u32           _pad;
        __u64           chunks;
};

allows userspace to pass in array of pointers to extension structs, but
must therefore keep constructing that array along side the command stream.
In dynamic situations like that, a linked list is preferred and does not
similar from extra cache line misses as the extension structs themselves
must still be loaded separate to the chunks array.

v2: Apply the tail call optimisation directly to nip the worry of stack
overflow in the bud.
v3: Defend against recursion.
v4: Fixup local types to match new uabi

Opens:
- do we include the result as an out-field in each chain?
struct i915_user_extension {
	__u64 next_extension;
	__u64 name;
	__s32 result;
	__u32 mbz; /* reserved for future use */
};
* Undecided, so provision some room for future expansion.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190322092325.5883-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_utils.h

@@ -105,6 +105,37 @@
 	__T;								\
 })
 
+/*
+ * container_of_user: Extract the superclass from a pointer to a member.
+ *
+ * Exactly like container_of() with the exception that it plays nicely
+ * with sparse for __user @ptr.
+ */
+#define container_of_user(ptr, type, member) ({				\
+	void __user *__mptr = (void __user *)(ptr);			\
+	BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) &&	\
+			 !__same_type(*(ptr), void),			\
+			 "pointer type mismatch in container_of()");	\
+	((type __user *)(__mptr - offsetof(type, member))); })
+
+/*
+ * check_user_mbz: Check that a user value exists and is zero
+ *
+ * Frequently in our uABI we reserve space for future extensions, and
+ * two ensure that userspace is prepared we enforce that space must
+ * be zero. (Then any future extension can safely assume a default value
+ * of 0.)
+ *
+ * check_user_mbz() combines checking that the user pointer is accessible
+ * and that the contained value is zero.
+ *
+ * Returns: -EFAULT if not accessible, -EINVAL if !zero, or 0 on success.
+ */
+#define check_user_mbz(U) ({						\
+	typeof(*(U)) mbz__;						\
+	get_user(mbz__, (U)) ? -EFAULT : mbz__ ? -EINVAL : 0;		\
+})
+
 static inline u64 ptr_to_u64(const void *ptr)
 {
 	return (uintptr_t)ptr;