[PATCH 3/4] x86/jump_label: Implement arch_static_assert()

[Peter Zijlstra]

Implement the static (branch) assertion. It simply emits the address
of the next instruction into a special section which objtool will read
and validate against either __jump_table or .altinstructions.

Use like:

	if (static_branch_likely(_key)) {
		arch_static_assert();
		/* do stuff */
	}

Or

	if (static_cpu_has(_feat)) {
		arch_static_assert();
		/* do stuff */
	}

Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/include/asm/jump_label.h |    9 +++++++++
 1 file changed, 9 insertions(+)

--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -62,6 +62,15 @@ static __always_inline bool arch_static_
 	return true;
 }
 
+static __always_inline void arch_static_assert(void)
+{
+	asm volatile ("1:\n\t"
+		      ".pushsection .discard.jump_assert, \"aw\" \n\t"
+		      _ASM_ALIGN  "\n\t"
+		      _ASM_PTR "1b \n\t"
+		      ".popsection \n\t");
+}
+
 #ifdef CONFIG_X86_64
 typedef u64 jump_label_t;
 #else