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The inline asm in __write_64bit_c0_split() modifies the 64-bit input operand by shifting the high register left by 32, and constructing the full 64-bit value in the low register (even on a 32-bit kernel), so if that value is used again it could cause breakage as GCC would assume the registers haven't changed when they have. To quote the GCC extended asm documentation: > Warning: Do not modify the contents of input-only operands (except for > inputs tied to outputs). The compiler assumes that on exit from the > asm statement these operands contain the same values as they had > before executing the statement. Avoid modifying the input by using a temporary variable as an output which is modified instead of the input and not otherwise used. The asm is always __volatile__ so GCC shouldn't optimise it out. The low register of the temporary output is written before the high register of the input is read, so we have two constraint alternatives, one where both use the same registers (for when the input value isn't subsequently used), and one with an early clobber on the output in case the low output uses the same register as the high input. This allows the resulting assembly to remain mostly unchanged. A diff of a MIPS32r6 kernel reveals only three differences, two in relation to write_c0_r10k_diag() in cpu_probe() (register allocation rearranged slightly but otherwise identical), and one in relation to write_c0_cvmmemctl2() in kvm_vz_local_flush_guesttlb_all(), but the octeon CPU is only supported on 64-bit kernels where __write_64bit_c0_split() isn't used so that shouldn't matter in practice. So there currently doesn't appear to be anything broken by this bug. Signed-off-by: James Hogan <james.hogan@imgtec.com> Cc: linux-mips@linux-mips.org Patchwork: https://patchwork.linux-mips.org/patch/17315/ Signed-off-by: Ralf Baechle <ralf@linux-mips.org> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.