Date: Fri, 24 Jul 2015 12:36:09 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Borislav Petkov <bp@alien8.de>
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
Message-ID: <20150724103609.GE19282@twins.programming.kicks-ass.net>
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 <20150723104215.GH25159@twins.programming.kicks-ass.net>
 <20150723113450.67db0152@gandalf.local.home>
 <20150723170811.GJ25159@twins.programming.kicks-ass.net>
 <20150723175436.GA12218@nazgul.tnic>
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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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