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

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Message-ID: <20150724105602.GF19282@twins.programming.kicks-ass.net>
Date:	Fri, 24 Jul 2015 12:56:02 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Baron <jasonbaron0@...il.com>
Cc:	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>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: Kernel broken on processors without performance counters

On Thu, Jul 23, 2015 at 03:14:13PM -0400, Jason Baron wrote:
> > +static enum jump_label_type jump_label_type(struct jump_entry *entry)
> > +{
> > +	struct static_key *key = static_key_cast(iter->key);
> > +	bool true_branch = jump_label_get_branch_default(key);
> > +	bool state = static_key_enabled(key);
> > +	bool inv = static_key_inv(iter->key);
> > +
> > +	return (true_branch ^ state) ^ inv;
> 
> iiuc...this allows both the old style keys to co-exist with the
> new ones. IE state wouldn't be set for the new ones. And inv
> shouldn't be set for the old ones. Is that correct?

@state is the dynamic variable here, the other two are compile time.
@true_branch denotes the default (compile time) value, and @inv denotes
the (compile time) branch preference.

So @state will still be set for the new ones, but you're right in that
@inv will not be set for the 'legacy' stuff.

This one little line needs a big comment.

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