linux-kernel - Re: Kernel broken on processors without performance counters

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Message-ID: <20150723141247.32d760eb@gandalf.local.home>
Date:	Thu, 23 Jul 2015 14:12:47 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jason Baron <jasonbaron0@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Kees Cook <keescook@...omium.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Vince Weaver <vince@...ter.net>,
	"hillf.zj" <hillf.zj@...baba-inc.com>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Kernel broken on processors without performance counters

On Thu, 23 Jul 2015 13:33:36 -0400
Jason Baron <jasonbaron0@...il.com> wrote:

> On 07/23/2015 01:08 PM, Peter Zijlstra wrote:
> > On Thu, Jul 23, 2015 at 11:34:50AM -0400, Steven Rostedt wrote:
> >> On Thu, 23 Jul 2015 12:42:15 +0200
> >> Peter Zijlstra <peterz@...radead.org> wrote:
> >>
> >>> static __always_inline bool arch_static_branch_jump(struct static_key *key, bool inv)
> >>> {
> >>> 	if (!inv) {
> >>> 		asm_volatile_goto("1:"
> >>> 			"jmp %l[l_yes]\n\t"
> >> And what happens when this gets converted to a two byte jump?
> >>
> > That would be bad, how can we force it to emit 5 bytes?
> hmm....I don't think that's an issue, the patching code can
> detect if its a 2-byte jump - 0xeb, or 5-byte: 0xe9, and do
> the correct no-op. Same going the other way. See the code
> I posted a few mails back. In fact, this gets us to the
> smaller 2-byte no-ops in cases where we are initialized
> to jump.
>

Ah right, and I already have the code that checks that (from the
original plan).

-- Steve
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