mm: gmem: Introduce GMEM

euleros inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7WLVX



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The functions of the GMEM can be summarized as follows:
First, the accelerator driver can directly reuse the core VM code of
Linux by providing the MMU operation function required by the GMEM,
and no independent memory management mechanism is required.

Second, the GMEM can coordinate a page table between multiple
heterogeneous MMUs, so as to implement memory coherence
(memory coherence) between the CPU and the accelerator in a same
address space. From a kernel point of view, the driver code for
memory management with repetitive functions is greatly reduced.

From the perspective of driver programming, the development and
maintenance workload of driver code is greatly reduced. From the
perspective of application development, the same address space
greatly reduces programming complexity, while GMEM provides
heterogeneous memory semantics to enhance flexibility and
ease of use in performance tuning.

To enable gmem, add "gmem=on" in kernel commandline.

Co-developed-by: Yang Yanchao <yangyanchao6@huawei.com>
Co-developed-by: luochunsheng <luochunsheng@huawei.com>
Co-developed-by: Weixi Zhu <weixi.zhu@huawei.com>
Signed-off-by: Bin Wang <wangbin224@huawei.com>