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Today the fixmap code largely maps elements at PAGE_SIZE granularity, but we special-case the FDT mapping such that it can be mapped with 2M block mappings when 4K pages are in use. The original rationale for this was simplicity, but it has some unfortunate side-effects, and complicates portions of the fixmap code (i.e. is not so simple after all). The FDT can be up to 2M in size but is only required to have 8-byte alignment, and so it may straddle a 2M boundary. Thus when using 2M block mappings we may map up to 4M of memory surrounding the FDT. This is unfortunate as most of that memory will be unrelated to the FDT, and any pages which happen to share a 2M block with the FDT will by mapped with Normal Write-Back Cacheable attributes, which might not be what we want elsewhere (e.g. for carve-outs using Non-Cacheable attributes). The logic to handle mapping the FDT with 2M blocks requires some special cases in the fixmap code, and ties it to the early page table configuration by virtue of the SWAPPER_TABLE_SHIFT and SWAPPER_BLOCK_SIZE constants used to determine the granularity used to map the FDT. This patch simplifies the FDT logic and removes the unnecessary mappings of surrounding pages by always mapping the FDT at page granularity as with all other fixmap mappings. To do so we statically reserve multiple PTE tables to cover the fixmap VA range. Since the FDT can be at most 2M, for 4K pages we only need to allocate a single additional PTE table, and for 16K and 64K pages the existing single PTE table is sufficient. The PTE table allocation scales with the number of slots reserved in the fixmap, and so this also makes it easier to add more fixmap entries if we require those in future. Our VA layout means that the fixmap will always fall within a single PMD table (and consequently, within a single PUD/P4D/PGD entry), which we can verify at compile time with a static_assert(). With that assert a number of runtime warnings become impossible, and are removed. I've boot-tested this patch with both 4K and 64K pages. Signed-off-by: Mark Rutland <mark.rutland@arm.com> Cc: Anshuman Khandual <anshuman.khandual@arm.com> Cc: Ard Biesheuvel <ardb@kernel.org> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Ryan Roberts <ryan.roberts@arm.com> Cc: Will Deacon <will@kernel.org> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com> Link: https://lore.kernel.org/r/20230406152759.4164229-4-mark.rutland@arm.com Signed-off-by: Will Deacon <will@kernel.org> |
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| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
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| tools | ||
| usr | ||
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| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
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| .gitignore | ||
| .mailmap | ||
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| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.