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

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Date:	Fri, 24 Jul 2015 12:36:09 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Jason Baron <jasonbaron0@...il.com>,
	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 Fri, Jul 24, 2015 at 07:29:05AM +0200, Borislav Petkov wrote:
> On Thu, Jul 23, 2015 at 09:02:14PM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 23, 2015 at 07:54:36PM +0200, Borislav Petkov wrote:
> > > On Thu, Jul 23, 2015 at 07:08:11PM +0200, Peter Zijlstra wrote:
> > > > That would be bad, how can we force it to emit 5 bytes?
> > > 
> > > .byte 0xe9 like we used to do in static_cpu_has_safe().
> > 
> > Like so then?
> > 
> > static __always_inline bool arch_static_branch_jump(struct static_key *key, bool inv)
> > {
> > 	unsigned long kval = (unsigned long)key + inv;
> > 
> > 	asm_volatile_goto("1:"
> > 		".byte 0xe9\n\t .long %l[l_yes]\n\t"
> > 		".pushsection __jump_table,  \"aw\" \n\t"
> > 		_ASM_ALIGN "\n\t"
> > 		_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
> > 		".popsection \n\t"
> > 		: :  "i" (kval) : : l_yes);
> > 
> > 	return false;
> > l_yes:
> > 	return true;
> > }
> 
> Yap.
> 
> But, we can do even better and note down what kind of JMP the compiler
> generated and teach __jump_label_transform() to generate the right one.
> Maybe this struct jump_entry would get a flags member or so. This way
> we're optimal.
> 
> Methinks...

Yes, Jason and Steve already have patches to go do that, but I'd really
like to keep that separate for now, this thing is big enough as is.
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