Message-ID: <55B12570.4070709@gmail.com>
Date: Thu, 23 Jul 2015 13:33:36 -0400
From: Jason Baron <jasonbaron0@gmail.com>
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To: Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>
CC: Borislav Petkov <bp@alien8.de>, 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
References: <alpine.DEB.2.11.1507211726040.18576@nanos> <20150721154959.GS19282@twins.programming.kicks-ass.net> <CALCETrWwy777nYTVpOn8Hs1JRG5nDyvky9B11otDRAid1L19vg@mail.gmail.com> <20150721161215.GU19282@twins.programming.kicks-ass.net> <20150721181553.GA3378@nazgul.tnic> <55AE9471.1000601@gmail.com> <20150722042403.GA6345@nazgul.tnic> <55AFCDA4.5010406@gmail.com> <20150723104215.GH25159@twins.programming.kicks-ass.net> <20150723113450.67db0152@gandalf.local.home> <20150723170811.GJ25159@twins.programming.kicks-ass.net>
In-Reply-To: <20150723170811.GJ25159@twins.programming.kicks-ass.net>
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On 07/23/2015 01:08 PM, Peter Zijlstra wrote:
> On Thu, Jul 23, 2015 at 11:34:50AM -0400, Steven Rostedt wrote:
>> On Thu, 23 Jul 2015 12:42:15 +0200
>> Peter Zijlstra <peterz@infradead.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?
>>
> That would be bad, how can we force it to emit 5 bytes?
hmm....I don't think that's an issue, the patching code can
detect if its a 2-byte jump - 0xeb, or 5-byte: 0xe9, and do
the correct no-op. Same going the other way. See the code
I posted a few mails back. In fact, this gets us to the
smaller 2-byte no-ops in cases where we are initialized
to jump.

Thanks,

-Jason
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