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arm64: simplify kernel segment mapping granularity
The mapping of the kernel consist of four segments, each of which is mapped with different permission attributes and/or lifetimes. To optimize the TLB and translation table footprint, we define various opaque constants in the linker script that resolve to different aligment values depending on the page size and whether CONFIG_DEBUG_ALIGN_RODATA is set. Considering that - a 4 KB granule kernel benefits from a 64 KB segment alignment (due to the fact that it allows the use of the contiguous bit), - the minimum alignment of the .data segment is THREAD_SIZE already, not PAGE_SIZE (i.e., we already have padding between _data and the start of the .data payload in many cases), - 2 MB is a suitable alignment value on all granule sizes, either for mapping directly (level 2 on 4 KB), or via the contiguous bit (level 3 on 16 KB and 64 KB), - anything beyond 2 MB exceeds the minimum alignment mandated by the boot protocol, and can only be mapped efficiently if the physical alignment happens to be the same, we can simplify this by standardizing on 64 KB (or 2 MB) explicitly, i.e., regardless of granule size, all segments are aligned either to 64 KB, or to 2 MB if CONFIG_DEBUG_ALIGN_RODATA=y. This also means we can drop the Kconfig dependency of CONFIG_DEBUG_ALIGN_RODATA on CONFIG_ARM64_4K_PAGES. Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Signed-off-by: Will Deacon <will.deacon@arm.com>
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2 changed files with 16 additions and 11 deletions
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@ -59,7 +59,7 @@ config DEBUG_RODATA
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If in doubt, say Y
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config DEBUG_ALIGN_RODATA
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depends on DEBUG_RODATA && ARM64_4K_PAGES
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depends on DEBUG_RODATA
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bool "Align linker sections up to SECTION_SIZE"
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help
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If this option is enabled, sections that may potentially be marked as
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@ -63,14 +63,19 @@ PECOFF_FILE_ALIGNMENT = 0x200;
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#endif
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#if defined(CONFIG_DEBUG_ALIGN_RODATA)
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#define ALIGN_DEBUG_RO . = ALIGN(1<<SECTION_SHIFT);
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#define ALIGN_DEBUG_RO_MIN(min) ALIGN_DEBUG_RO
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#elif defined(CONFIG_DEBUG_RODATA)
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#define ALIGN_DEBUG_RO . = ALIGN(1<<PAGE_SHIFT);
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#define ALIGN_DEBUG_RO_MIN(min) ALIGN_DEBUG_RO
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/*
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* 4 KB granule: 1 level 2 entry
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* 16 KB granule: 128 level 3 entries, with contiguous bit
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* 64 KB granule: 32 level 3 entries, with contiguous bit
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*/
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#define SEGMENT_ALIGN SZ_2M
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#else
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#define ALIGN_DEBUG_RO
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#define ALIGN_DEBUG_RO_MIN(min) . = ALIGN(min);
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/*
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* 4 KB granule: 16 level 3 entries, with contiguous bit
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* 16 KB granule: 4 level 3 entries, without contiguous bit
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* 64 KB granule: 1 level 3 entry
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*/
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#define SEGMENT_ALIGN SZ_64K
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#endif
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SECTIONS
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@ -114,12 +119,12 @@ SECTIONS
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*(.got) /* Global offset table */
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}
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ALIGN_DEBUG_RO_MIN(PAGE_SIZE)
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. = ALIGN(SEGMENT_ALIGN);
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RO_DATA(PAGE_SIZE) /* everything from this point to */
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EXCEPTION_TABLE(8) /* _etext will be marked RO NX */
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NOTES
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ALIGN_DEBUG_RO_MIN(PAGE_SIZE)
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. = ALIGN(SEGMENT_ALIGN);
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_etext = .; /* End of text and rodata section */
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__init_begin = .;
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@ -168,7 +173,7 @@ SECTIONS
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*(.hash)
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}
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. = ALIGN(PAGE_SIZE);
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. = ALIGN(SEGMENT_ALIGN);
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__init_end = .;
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_data = .;
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