linux-kernel - RE: [PATCH] objtool,x86: Teach decode about LOOP* instructions

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Date:   Wed, 7 Sep 2022 09:06:12 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
CC:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        bpf <bpf@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Borislav Petkov" <bp@...e.de>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>
Subject: RE: [PATCH] objtool,x86: Teach decode about LOOP* instructions

From: Peter Zijlstra
> Sent: 07 September 2022 10:01
> 
> On Wed, Sep 07, 2022 at 09:06:45AM +0200, Peter Zijlstra wrote:
> > On Wed, Sep 07, 2022 at 09:55:21AM +0900, Masami Hiramatsu (Google) wrote:
> >
> > > +/* Return the jump target address or 0 */
> > > +static inline unsigned long insn_get_branch_addr(struct insn *insn)
> > > +{
> > > +	switch (insn->opcode.bytes[0]) {
> > > +	case 0xe0:	/* loopne */
> > > +	case 0xe1:	/* loope */
> > > +	case 0xe2:	/* loop */
> >
> > Oh cute, objtool doesn't know about those, let me go add them.

Do they ever appear in the kernel?
They are so slow on Intel cpu that finding one ought to
deemed a bug!

Have you got jcxz (0xe3) in there?
They are fast on both Intel and AMD cpus - so are usable.

	David

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