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b32fd8a60f
GCC fancies inlining memcpy(), and because it cannot prove the destination is page-aligned (it is) it ends up generating atrocious code like: 19e: 48 8b 15 00 00 00 00 mov 0x0(%rip),%rdx # 1a5 <relocate_restore_code+0x25> 1a1: R_X86_64_PC32 core_restore_code-0x4 1a5: 48 8d 78 08 lea 0x8(%rax),%rdi 1a9: 48 89 c1 mov %rax,%rcx 1ac: 48 c7 c6 00 00 00 00 mov $0x0,%rsi 1af: R_X86_64_32S core_restore_code 1b3: 48 83 e7 f8 and $0xfffffffffffffff8,%rdi 1b7: 48 29 f9 sub %rdi,%rcx 1ba: 48 89 10 mov %rdx,(%rax) 1bd: 48 8b 15 00 00 00 00 mov 0x0(%rip),%rdx # 1c4 <relocate_restore_code+0x44> 1c0: R_X86_64_PC32 core_restore_code+0xff4 1c4: 48 29 ce sub %rcx,%rsi 1c7: 81 c1 00 10 00 00 add $0x1000,%ecx 1cd: 48 89 90 f8 0f 00 00 mov %rdx,0xff8(%rax) 1d4: c1 e9 03 shr $0x3,%ecx 1d7: f3 48 a5 rep movsq %ds:(%rsi),%es:(%rdi) Notably the alignment code generates a text reference to code_restore_code+0xff8, for which objtool raises the objection: vmlinux.o: warning: objtool: relocate_restore_code+0x3d: relocation to !ENDBR: next_arg+0x18 Applying some __assume_aligned(PAGE_SIZE) improve code-gen to: 19e: 48 89 c7 mov %rax,%rdi 1a1: 48 c7 c6 00 00 00 00 mov $0x0,%rsi 1a4: R_X86_64_32S core_restore_code 1a8: b9 00 02 00 00 mov $0x200,%ecx 1ad: f3 48 a5 rep movsq %ds:(%rsi),%es:(%rdi) And resolve the problem, however, none of this is important code and a much simpler solution still is to force a memcpy() call: 1a1: ba 00 10 00 00 mov $0x1000,%edx 1a6: 48 c7 c6 00 00 00 00 mov $0x0,%rsi 1a9: R_X86_64_32S core_restore_code 1ad: e8 00 00 00 00 call 1b2 <relocate_restore_code+0x32> 1ae: R_X86_64_PLT32 __memcpy-0x4 Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.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 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.