Commit ab325704 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

jump_label, x86: Allow short NOPs 



Now that objtool is able to rewrite jump_label instructions, have the
compiler emit a JMP, such that it can decide on the optimal encoding,
and set jump_entry::key bit1 to indicate that objtool should rewrite
the instruction to a matching NOP.

For x86_64-allyesconfig this gives:

  jl\     NOP     JMP
  short:  22997   124
  long:   30874   90

IOW, we save (22997+124) * 3 bytes of kernel text in hotpaths.

Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210506194158.216763632@infradead.org
parent e2d9494b

arch/x86/include/asm/jump_label.h

+18 −0
Original line number Diff line number Diff line
@@ -20,6 +20,22 @@ 
	_ASM_PTR "%c0 + %c1 - .\n\t"			\

	".popsection \n\t"



#ifdef CONFIG_STACK_VALIDATION



static __always_inline bool arch_static_branch(struct static_key *key, bool branch)

{

	asm_volatile_goto("1:"

		"jmp %l[l_yes] # objtool NOPs this \n\t"

		JUMP_TABLE_ENTRY

		: :  "i" (key), "i" (2 | branch) : : l_yes);



	return false;

l_yes:

	return true;

}



#else



static __always_inline bool arch_static_branch(struct static_key * const key, const bool branch)

{

	asm_volatile_goto("1:"
@@ -32,6 +48,8 @@ static __always_inline bool arch_static_branch(struct static_key * const key, co 
	return true;

}



#endif /* STACK_VALIDATION */



static __always_inline bool arch_static_branch_jump(struct static_key * const key, const bool branch)

{

	asm_volatile_goto("1:"