linux-kernel - Re: [PATCH 1/2] objtool: Optimize re-writing jump

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Date:   Fri, 8 Oct 2021 12:03:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, mbenes@...e.cz
Subject: Re: [PATCH 1/2] objtool: Optimize re-writing jump_label

On Thu, Oct 07, 2021 at 11:55:50PM -0700, Josh Poimboeuf wrote:
> On Thu, Oct 07, 2021 at 11:22:12PM +0200, Peter Zijlstra wrote:
> > There's no point to re-write the jump_label NOP when it's already a NOP.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  tools/objtool/check.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > --- a/tools/objtool/check.c
> > +++ b/tools/objtool/check.c
> > @@ -1397,7 +1397,7 @@ static int handle_jump_alt(struct objtoo
> >  		return -1;
> >  	}
> >  
> > -	if (special_alt->key_addend & 2) {
> > +	if ((special_alt->key_addend & 2) && orig_insn->type != INSN_NOP) {
> >  		struct reloc *reloc = insn_reloc(file, orig_insn);
> >  
> >  		if (reloc) {
> 
> While you're at it, a comment would be very helpful for that whole
> clause.

Like so?

--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1397,7 +1397,16 @@ static int handle_jump_alt(struct objtoo
 		return -1;
 	}
 
-	if (special_alt->key_addend & 2) {
+	/*
+	 * When, for whatever reason, the jump-label site cannot emit a right
+	 * sized NOP, then it can use Bit-1 of the struct static_key pointer to
+	 * indicate this instruction should be NOP'ed by objtool.
+	 *
+	 * Also see arch/x86/include/asm/jump_label.h:arch_static_branch(),
+	 * where we leave the assembler to pick between jmp.d8 and jmp.d32
+	 * based on destination offset.
+	 */
+	if ((special_alt->key_addend & 2) && orig_insn->type != INSN_NOP) {
 		struct reloc *reloc = insn_reloc(file, orig_insn);
 
 		if (reloc) {