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

diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index ef819e3..0449b12 100644 (file)
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -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:"