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When booting in mixed mode, the firmware's GDT is still installed at handover entry in efi32_stub_entry. We save the GDTR for later use in __efi64_thunk but we are assuming that descriptor 2 (__KERNEL_CS) is a valid 32-bit code segment descriptor and that descriptor 3 (__KERNEL_DS/__BOOT_DS) is a valid data segment descriptor. This happens to be true for OVMF (it actually uses descriptor 1 for data segments, but descriptor 3 is also setup as data), but we shouldn't depend on this being the case. Fix this by saving the code and data selectors in addition to the GDTR in efi32_stub_entry, and restoring them in __efi64_thunk before calling the firmware. The UEFI specification guarantees that selectors will be flat, so using the DS selector for all the segment registers should be enough. We also need to install our own GDT before initializing segment registers in startup_32, so move the GDT load up to the beginning of the function. [ardb: mention mixed mode in the commit log] Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu> Link: https://lore.kernel.org/r/20200202171353.3736319-3-nivedita@alum.mit.edu Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
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.