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Use asm_inline() to instruct the compiler that the size of asm() is the minimum size of one instruction, ignoring how many instructions the compiler thinks it is. ALTERNATIVE macro that expands to several pseudo directives causes instruction length estimate to count more than 20 instructions. bloat-o-meter reports minimal code size increase (x86_64 defconfig with CONFIG_ADDRESS_MASKING, gcc-14.2.1): add/remove: 2/2 grow/shrink: 5/1 up/down: 2365/-1995 (370) Function old new delta ----------------------------------------------------- do_get_mempolicy - 1449 +1449 copy_nodes_to_user - 226 +226 __x64_sys_get_mempolicy 35 213 +178 syscall_user_dispatch_set_config 157 332 +175 __ia32_sys_get_mempolicy 31 206 +175 set_syscall_user_dispatch 29 181 +152 __do_sys_mremap 2073 2083 +10 sp_insert 133 117 -16 task_set_syscall_user_dispatch 172 - -172 kernel_get_mempolicy 1807 - -1807 Total: Before=21423151, After=21423521, chg +0.00% The code size increase is due to the compiler inlining more functions that inline untagged_addr(), e.g: task_set_syscall_user_dispatch() is now fully inlined in set_syscall_user_dispatch(): 000000000010b7e0 <set_syscall_user_dispatch>: 10b7e0: f3 0f 1e fa endbr64 10b7e4: 49 89 c8 mov %rcx,%r8 10b7e7: 48 89 d1 mov %rdx,%rcx 10b7ea: 48 89 f2 mov %rsi,%rdx 10b7ed: 48 89 fe mov %rdi,%rsi 10b7f0: 65 48 8b 3d 00 00 00 mov %gs:0x0(%rip),%rdi 10b7f7: 00 10b7f8: e9 03 fe ff ff jmp 10b600 <task_set_syscall_user_dispatch> that after inlining becomes: 000000000010b730 <set_syscall_user_dispatch>: 10b730: f3 0f 1e fa endbr64 10b734: 65 48 8b 05 00 00 00 mov %gs:0x0(%rip),%rax 10b73b: 00 10b73c: 48 85 ff test %rdi,%rdi 10b73f: 74 54 je 10b795 <set_syscall_user_dispatch+0x65> 10b741: 48 83 ff 01 cmp $0x1,%rdi 10b745: 74 06 je 10b74d <set_syscall_user_dispatch+0x1d> 10b747: b8 ea ff ff ff mov $0xffffffea,%eax 10b74c: c3 ret 10b74d: 48 85 f6 test %rsi,%rsi 10b750: 75 7b jne 10b7cd <set_syscall_user_dispatch+0x9d> 10b752: 48 85 c9 test %rcx,%rcx 10b755: 74 1a je 10b771 <set_syscall_user_dispatch+0x41> 10b757: 48 89 cf mov %rcx,%rdi 10b75a: 49 b8 ef cd ab 89 67 movabs $0x123456789abcdef,%r8 10b761: 45 23 01 10b764: 90 nop 10b765: 90 nop 10b766: 90 nop 10b767: 90 nop 10b768: 90 nop 10b769: 90 nop 10b76a: 90 nop 10b76b: 90 nop 10b76c: 49 39 f8 cmp %rdi,%r8 10b76f: 72 6e jb 10b7df <set_syscall_user_dispatch+0xaf> 10b771: 48 89 88 48 08 00 00 mov %rcx,0x848(%rax) 10b778: 48 89 b0 50 08 00 00 mov %rsi,0x850(%rax) 10b77f: 48 89 90 58 08 00 00 mov %rdx,0x858(%rax) 10b786: c6 80 60 08 00 00 00 movb $0x0,0x860(%rax) 10b78d: f0 80 48 08 20 lock orb $0x20,0x8(%rax) 10b792: 31 c0 xor %eax,%eax 10b794: c3 ret 10b795: 48 09 d1 or %rdx,%rcx 10b798: 48 09 f1 or %rsi,%rcx 10b79b: 75 aa jne 10b747 <set_syscall_user_dispatch+0x17> 10b79d: 48 c7 80 48 08 00 00 movq $0x0,0x848(%rax) 10b7a4: 00 00 00 00 10b7a8: 48 c7 80 50 08 00 00 movq $0x0,0x850(%rax) 10b7af: 00 00 00 00 10b7b3: 48 c7 80 58 08 00 00 movq $0x0,0x858(%rax) 10b7ba: 00 00 00 00 10b7be: c6 80 60 08 00 00 00 movb $0x0,0x860(%rax) 10b7c5: f0 80 60 08 df lock andb $0xdf,0x8(%rax) 10b7ca: 31 c0 xor %eax,%eax 10b7cc: c3 ret 10b7cd: 48 8d 3c 16 lea (%rsi,%rdx,1),%rdi 10b7d1: 48 39 fe cmp %rdi,%rsi 10b7d4: 0f 82 78 ff ff ff jb 10b752 <set_syscall_user_dispatch+0x22> 10b7da: e9 68 ff ff ff jmp 10b747 <set_syscall_user_dispatch+0x17> 10b7df: b8 f2 ff ff ff mov $0xfffffff2,%eax 10b7e4: c3 ret Please note a series of NOPs that get replaced with an alternative: 11f0: 65 48 23 05 00 00 00 and %gs:0x0(%rip),%rax 11f7: 00 Signed-off-by: Uros Bizjak <ubizjak@gmail.com> Signed-off-by: Ingo Molnar <mingo@kernel.org> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Link: https://lore.kernel.org/r/20250407072129.33440-1-ubizjak@gmail.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 reStructuredText 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.