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016919c1f2
Linus suggested replacing the magic numbers in the GDT descriptors using preprocessor macros. Designing the interface properly is actually pretty hard -- there are several constraints: - you want the final expressions to be readable at a glance; something like GDT_ENTRY_FLAGS(5, 1, 0, 1, 0, 1, 1, 0) isn't because you need to visit the definition to understand what each parameter represents and then match up parameters in the user and the definition (which is hard when there are so many of them) - you want the final expressions to be fairly short/information-dense; something like GDT_ENTRY_PRESENT | GDT_ENTRY_DATA_WRITABLE | GDT_ENTRY_SYSTEM | GDT_ENTRY_DB | GDT_ENTRY_GRANULARITY_4K is a bit too verbose to write out every time and is actually hard to read as well because of all the repetition - you may want to assume defaults for some things (e.g. entries are DPL-0 a.k.a. kernel segments by default) and allow the user to override the default -- but this works best if you can OR in the override; if you want DPL-3 by default and override with DPL-0 you would need to start masking off bits instead of OR-ing them in and that just becomes harder to read - you may want to parameterize some things (e.g. CODE vs. DATA or KERNEL vs. USER) since both values are used and you don't really want prefer either one by default -- or DPL, which is always some value that is always specified This patch tries to balance these requirements and has two layers of definitions -- low-level and high-level: - the low-level defines are the mapping between human-readable names and the actual bit numbers - the high-level defines are the mapping from high-level intent to combinations of low-level flags, representing roughly a tuple (data/code/tss, 64/32/16-bits) plus an override for DPL-3 (= USER), since that's relatively rare but still very important to mark properly for those segments. - we have *_BIOS variants for 32-bit code and data segments that don't have the G flag set and give the limit in terms of bytes instead of pages [ mingo: Improved readability bit more. ] Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com> Signed-off-by: Ingo Molnar <mingo@kernel.org> Acked-by: Linus Torvalds <torvalds@linux-foundation.org> Link: https://lore.kernel.org/r/20231219151200.2878271-2-vegard.nossum@oracle.com |
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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.