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36e9f71bd6
Add documentation on how the CXL driver interacts with the DAX driver. Signed-off-by: Gregory Price <gourry@gourry.net> Link: https://patch.msgid.link/20250512162134.3596150-12-gourry@gourry.net Signed-off-by: Dave Jiang <dave.jiang@intel.com>
44 lines
1.8 KiB
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44 lines
1.8 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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====================
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DAX Driver Operation
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====================
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The `Direct Access Device` driver was originally designed to provide a
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memory-like access mechanism to memory-like block-devices. It was
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extended to support CXL Memory Devices, which provide user-configured
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memory devices.
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The CXL subsystem depends on the DAX subsystem to either:
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- Generate a file-like interface to userland via :code:`/dev/daxN.Y`, or
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- Engage the memory-hotplug interface to add CXL memory to page allocator.
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The DAX subsystem exposes this ability through the `cxl_dax_region` driver.
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A `dax_region` provides the translation between a CXL `memory_region` and
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a `DAX Device`.
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DAX Device
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==========
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A `DAX Device` is a file-like interface exposed in :code:`/dev/daxN.Y`. A
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memory region exposed via dax device can be accessed via userland software
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via the :code:`mmap()` system-call. The result is direct mappings to the
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CXL capacity in the task's page tables.
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Users wishing to manually handle allocation of CXL memory should use this
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interface.
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kmem conversion
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===============
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The :code:`dax_kmem` driver converts a `DAX Device` into a series of `hotplug
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memory blocks` managed by :code:`kernel/memory-hotplug.c`. This capacity
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will be exposed to the kernel page allocator in the user-selected memory
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zone.
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The :code:`memmap_on_memory` setting (both global and DAX device local)
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dictates where the kernell will allocate the :code:`struct folio` descriptors
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for this memory will come from. If :code:`memmap_on_memory` is set, memory
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hotplug will set aside a portion of the memory block capacity to allocate
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folios. If unset, the memory is allocated via a normal :code:`GFP_KERNEL`
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allocation - and as a result will most likely land on the local NUM node of the
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CPU executing the hotplug operation.
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