From: David Laight <David.Laight@ACULAB.COM>
To: 'Peter Zijlstra' <peterz@infradead.org>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>,
Steven Rostedt <rostedt@goodmis.org>,
Ingo Molnar <mingo@kernel.org>,
Suleiman Souhlal <suleiman@google.com>, bpf <bpf@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"Borislav Petkov" <bp@suse.de>,
Josh Poimboeuf <jpoimboe@kernel.org>,
"x86@kernel.org" <x86@kernel.org>
Subject: RE: [PATCH] objtool,x86: Teach decode about LOOP* instructions
Date: Wed, 7 Sep 2022 11:13:54 +0000 [thread overview]
Message-ID: <7889af4b7bb84823aca1732fb0d14de5@AcuMS.aculab.com> (raw)
In-Reply-To: <Yxhm9HuSKSjznSzP@hirez.programming.kicks-ass.net>
From: Peter Zijlstra
> Sent: 07 September 2022 10:40
>
> On Wed, Sep 07, 2022 at 09:06:12AM +0000, David Laight wrote:
> > 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?
>
> No; that is, not on any of the random vmlinux.o images I checked this
> morning.
>
> Still, best to properly decode them anyway.
It is annoying that cpu with adox/adcx have slow loop.
You really want to be able to do:
1: adox ...
adcx ...
loop 1b
That would never run with one iteration/clock.
But unrolling once would probably be enough.
What you can do (and gives the fastest IPcsum loop) is:
1: jcxz 2f
....
lea %rcx,...
jmp 1b
2:
The extra instructions mean that needs unrolling 4 times.
I've got over 12 bytes/clock that way.
David
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Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2022-09-07 11:14 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-07 0:55 [PATCH 0/2] x86/kprobes: Fixes for CONFIG_RETHUNK Masami Hiramatsu (Google)
2022-09-07 0:55 ` [PATCH 1/2] x86/kprobes: Fix kprobes instruction boudary check with CONFIG_RETHUNK Masami Hiramatsu (Google)
2022-09-07 7:06 ` Peter Zijlstra
2022-09-07 9:01 ` [PATCH] objtool,x86: Teach decode about LOOP* instructions Peter Zijlstra
2022-09-07 9:06 ` David Laight
2022-09-07 9:40 ` Peter Zijlstra
2022-09-07 11:13 ` David Laight [this message]
2022-09-15 14:24 ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2022-09-07 9:12 ` [PATCH 1/2] x86/kprobes: Fix kprobes instruction boudary check with CONFIG_RETHUNK Masami Hiramatsu
2022-09-07 9:38 ` Peter Zijlstra
2022-09-07 9:53 ` Masami Hiramatsu
2022-09-07 8:02 ` Peter Zijlstra
2022-09-07 8:11 ` Peter Zijlstra
2022-09-07 9:49 ` Masami Hiramatsu
2022-09-07 10:19 ` Peter Zijlstra
2022-09-07 11:44 ` Peter Zijlstra
2022-09-07 13:05 ` Peter Zijlstra
2022-09-07 14:14 ` Masami Hiramatsu
2022-09-07 14:27 ` Peter Zijlstra
2022-09-07 15:22 ` Masami Hiramatsu
2022-09-07 12:56 ` Peter Zijlstra
2022-09-07 13:49 ` Masami Hiramatsu
2022-09-07 14:28 ` Peter Zijlstra
2022-09-07 12:59 ` Peter Zijlstra
2022-09-07 13:53 ` Masami Hiramatsu
2022-09-07 0:55 ` [PATCH 2/2] x86/kprobes: Fix optprobe optimization " Masami Hiramatsu (Google)
2022-09-07 6:52 ` [PATCH 0/2] x86/kprobes: Fixes for CONFIG_RETHUNK Peter Zijlstra
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