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

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Message-ID: <CALCETrUYnddMbS3Yx=MV+_98tQ7ibKDBw1trpssJGo6D4W_VFw@mail.gmail.com>
Date:	Tue, 21 Jul 2015 11:54:30 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Jason Baron <jasonbaron0@...il.com>
Cc:	Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>,
	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>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: Kernel broken on processors without performance counters

On Tue, Jul 21, 2015 at 11:50 AM, Jason Baron <jasonbaron0@...il.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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