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Date: Fri, 24 Jul 2015 07:29:05 +0200
From: Borislav Petkov <bp@alien8.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>, Jason Baron <jasonbaron0@gmail.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mikulas Patocka <mpatocka@redhat.com>,
        Paul Mackerras <paulus@samba.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Vince Weaver <vince@deater.net>, "hillf.zj" <hillf.zj@alibaba-inc.com>,
        Valdis Kletnieks <Valdis.Kletnieks@vt.edu>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: Kernel broken on processors without performance counters
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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...

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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