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Currently the PTE_PRESENT_INVALID and PTE_PROT_NONE functionality
explicitly occupy 2 bits in the PTE when PTE_VALID/PMD_SECT_VALID is
clear. This has 2 significant consequences:
- PTE_PROT_NONE consumes a precious SW PTE bit that could be used for
other things.
- The swap pte layout must reserve those same 2 bits and ensure they
are both always zero for a swap pte. It would be nice to reclaim at
least one of those bits.
But PTE_PRESENT_INVALID, which since the previous patch, applies
uniformly to page/block descriptors at any level when PTE_VALID is
clear, can already give us most of what PTE_PROT_NONE requires: If it is
set, then the pte is still considered present; pte_present() returns
true and all the fields in the pte follow the HW interpretation (e.g. SW
can safely call pte_pfn(), etc). But crucially, the HW treats the pte as
invalid and will fault if it hits.
So let's remove PTE_PROT_NONE entirely and instead represent PROT_NONE
as a present but invalid pte (PTE_VALID=0, PTE_PRESENT_INVALID=1) with
PTE_USER=0 and PTE_UXN=1. This is a unique combination that is not used
anywhere else.
The net result is a clearer, simpler, more generic encoding scheme that
applies uniformly to all levels. Additionally we free up a PTE SW bit
and a swap pte bit (bit 58 in both cases).
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Link: https://lore.kernel.org/r/20240503144604.151095-3-ryan.roberts@arm.com
Signed-off-by: Will Deacon <will@kernel.org>
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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 reStructuredText 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.