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

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Date:	Thu, 23 Jul 2015 11:34:50 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jason Baron <jasonbaron0@...il.com>,
	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 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?

-- Steve

> 			".pushsection __jump_table,  \"aw\" \n\t"
> 			_ASM_ALIGN "\n\t"
> 			_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
> 			".popsection \n\t"
> 			: :  "i" (key) : : l_yes);
> 	} else {
> 		asm_volatile_goto("1:"
> 			"jmp %l[l_yes]\n\t"
> 			".pushsection __jump_table_inv,  \"aw\" \n\t"
> 			_ASM_ALIGN "\n\t"
> 			_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
> 			".popsection \n\t"
> 			: :  "i" (key) : : l_yes);
> 	}
> 	return false;
> l_yes:
> 	return true;
> }
> 
> 
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