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doc: guides: x86: how to specify extra page mappings

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This adds to page on the x86 virtual memory about how to specify
extra page table mappings during build.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>
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Daniel Leung 2021-03-15 12:36:27 -07:00 • committed by Anas Nashif
commit 7ce8c77650
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doc/guides/arch/x86.rst
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@ -76,3 +76,51 @@ where virtual address space can be:
- Due to re-using of second level entries, both
``CONFIG_SRAM_OFFSET`` and ``CONFIG_KERNEL_VM_OFFSET`` must be of
same value.
Specifying Additional Memory Mappings at Build Time
***************************************************
The page table generation script (:file:`arch/x86/gen_mmu.py`) generates
the necessary multi-level page tables for code execution and data access
using the kernel image produced by the first linker pass. Additional
command line arguments can be passed to the script to generate additional
memory mappings. This is useful for static mappings and/or device MMIO
access during very early boot. To pass extra command line arguments to
the script, populate a CMake list named ``X86_EXTRA_GEN_MMU_ARGUMENTS``
in the board configuration file. Here is an example:
.. code-block:: cmake
set(X86_EXTRA_GEN_MMU_ARGUMENTS
--map 0xA0000000,0x2000
--map 0x80000000,0x400000,LWUX,0xB0000000)
The argument ``--map`` takes the following value:
``<physical address>,<size>[,<flags:LUWX>[,<virtual adderss>]]``, where:
- ``<physical address>`` is the physical address of the mapping. (Required)
- ``<size>`` is the size of the region to be mapped. (Required)
- ``<flags>`` is the flag associated with the mapping: (Optional)
- ``L``: Large page at the page directory level.
- ``U``: Allow userspace access.
- ``W``: Read/write.
- ``X``: Allow execution.
- Default is small page (4KB), supervisor only, read only, and
execution disabled.
- ``<virtual address`` is the virtual address of the mapping. (Optional)
Note that specifying additional memory mappings requires larger storage
space for the pre-allocated page tables (both kernel and per-domain
tables). :option:`CONFIG_X86_EXTRA_PAGE_TABLE_PAGES` is needed to
specify how many more memory pages to be reserved for the page tables.
If the needed space is not exactly the same as required space,
the ``gen_mmu.py`` script will print out a message indicating what
needs to be the value for the kconfig.