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From: Andy Lutomirski <luto@amacapital.net>
Date: Tue, 21 Jul 2015 11:54:30 -0700
Message-ID: <CALCETrUYnddMbS3Yx=MV+_98tQ7ibKDBw1trpssJGo6D4W_VFw@mail.gmail.com>
Subject: Re: Kernel broken on processors without performance counters
To: Jason Baron <jasonbaron0@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>, Peter Zijlstra <peterz@infradead.org>,
        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>,
        Steven Rostedt <rostedt@goodmis.org>
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On Tue, Jul 21, 2015 at 11:50 AM, Jason Baron <jasonbaron0@gmail.com> wrote:
>
>
> On 07/21/2015 02:15 PM, Borislav Petkov wrote:
>> On Tue, Jul 21, 2015 at 06:12:15PM +0200, Peter Zijlstra wrote:
>>> Yes, if you start out false, you must be unlikely. If you start out
>>> true, you must be likely.
>>>
>>> We could maybe try and untangle that if there really is a good use case,
>>> but this is the current state.
>>>
>>> The whole reason this happened is because 'false' is like:
>>>
>>>
>>>      ...
>>>      <nop>
>>> 1:
>>>      ...
>>>
>>>
>>>
>>> label:
>>>      <unlikely code>
>>>      jmp 1b
>>>
>>>
>>> Where the code if out-of-line by default. The enable will rewrite the
>>> <nop> with a jmp label.
>> Btw, native_sched_clock() is kinda botched because of that, see below.
>>
>> I'd want that RDTSC to come first with a NOP preceding it which can
>> become a JMP in case some idiotic CPU can't do RDTSC and needs to use
>> jiffies. Instead, we *unconditionally* jump to RDTSC which is WTF?! We
>> can just as well do a normal unlikely() without the static_key:
>
> hmmm...so this is a case where need to the default the branch
> to the out-of-line branch at boot. That is, we can't just enable
> the out-of-line branch at boot time, b/c it might be too late at
> that point? IE native_sched_clock() gets called very early?
>

Could this be done at link time, or perhaps when compressing the
kernel image, instead of at boot time?

--Andy
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