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This prepare kernel to operate with a different value than userspace AMR/IAMR. For this, AMR/IAMR need to be saved and restored on entry and return from the kernel. With KUAP we modify kernel AMR when accessing user address from the kernel via copy_to/from_user interfaces. We don't need to modify IAMR value in similar fashion. If MMU_FTR_PKEY is enabled we need to save AMR/IAMR in pt_regs on entering kernel from userspace. If not we can assume that AMR/IAMR is not modified from userspace. We need to save AMR if we have MMU_FTR_BOOK3S_KUAP feature enabled and we are interrupted within kernel. This is required so that if we get interrupted within copy_to/from_user we continue with the right AMR value. If we hae MMU_FTR_BOOK3S_KUEP enabled we need to restore IAMR on return to userspace beause kernel will be running with a different IAMR value. Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com> Reviewed-by: Sandipan Das <sandipan@linux.ibm.com> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> Link: https://lore.kernel.org/r/20201127044424.40686-11-aneesh.kumar@linux.ibm.com
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.