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Currently, kernel fails to boot on some HyperV VMs when using EFI. And it's a potential issue on all x86 platforms. It's caused by broken kernel relocation on EFI systems, when below three conditions are met: 1. Kernel image is not loaded to the default address (LOAD_PHYSICAL_ADDR) by the loader. 2. There isn't enough room to contain the kernel, starting from the default load address (eg. something else occupied part the region). 3. In the memmap provided by EFI firmware, there is a memory region starts below LOAD_PHYSICAL_ADDR, and suitable for containing the kernel. EFI stub will perform a kernel relocation when condition 1 is met. But due to condition 2, EFI stub can't relocate kernel to the preferred address, so it fallback to ask EFI firmware to alloc lowest usable memory region, got the low region mentioned in condition 3, and relocated kernel there. It's incorrect to relocate the kernel below LOAD_PHYSICAL_ADDR. This is the lowest acceptable kernel relocation address. The first thing goes wrong is in arch/x86/boot/compressed/head_64.S. Kernel decompression will force use LOAD_PHYSICAL_ADDR as the output address if kernel is located below it. Then the relocation before decompression, which move kernel to the end of the decompression buffer, will overwrite other memory region, as there is no enough memory there. To fix it, just don't let EFI stub relocate the kernel to any address lower than lowest acceptable address. [ ardb: introduce efi_low_alloc_above() to reduce the scope of the change ] Signed-off-by: Kairui Song <kasong@redhat.com> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Acked-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-efi@vger.kernel.org Link: https://lkml.kernel.org/r/20191029173755.27149-6-ardb@kernel.org Signed-off-by: Ingo Molnar <mingo@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.