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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: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210506194158.216763632@infradead.org
This commit is contained in:
Peter Zijlstra 2021-05-06 21:34:05 +02:00 committed by Ingo Molnar
parent e2d9494bef
commit ab3257042c
1 changed files with 18 additions and 0 deletions
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18
arch/x86/include/asm/jump_label.h
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@ -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 @@ l_yes:
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:"